fix(startup): print --listen address on failure

Before:

    $ nvim --headless --listen ./hello.sock
    nvim: Failed to --listen: "address already in use"

After:

    $ nvim --headless --listen ./hello.sock
    nvim: Failed to --listen: "./hello.sock": "address already in use"

src/nvim/main.c

@@ -266,7 +266,7 @@ int main(int argc, char **argv)
 
   if (argc > 1 && STRICMP(argv[1], "-ll") == 0) {
     if (argc == 2) {
-      print_mainerr(err_arg_missing, argv[1]);
+      print_mainerr(err_arg_missing, argv[1], NULL);
       exit(1);
     }
     nlua_run_script(argv, argc, 3);
@@ -359,8 +359,9 @@ int main(int argc, char **argv)
 
   int listen_rv = server_init(params.listen_addr);
   if (listen_rv != 0) {
-    mainerr("Failed to --listen", listen_rv < 0
-            ? os_strerror(listen_rv) : (listen_rv == 1 ? "empty address" : NULL));
+    mainerr("Failed to --listen",
+            params.listen_addr ? params.listen_addr : "(empty)",
+            listen_rv < 0 ? os_strerror(listen_rv) : (listen_rv == 1 ? "empty address" : NULL));
   }
 
   TIME_MSG("expanding arguments");
@@ -1053,7 +1054,7 @@ static void command_line_scan(mparm_T *parmp)
     // "+" or "+{number}" or "+/{pat}" or "+{command}" argument.
     if (argv[0][0] == '+' && !had_minmin) {
       if (parmp->n_commands >= MAX_ARG_CMDS) {
-        mainerr(err_extra_cmd, NULL);
+        mainerr(err_extra_cmd, NULL, NULL);
       }
       argv_idx = -1;  // skip to next argument
       if (argv[0][1] == NUL) {
@@ -1074,7 +1075,7 @@ static void command_line_scan(mparm_T *parmp)
           parmp->no_swap_file = true;
         } else {
           if (parmp->edit_type > EDIT_STDIN) {
-            mainerr(err_too_many_args, argv[0]);
+            mainerr(err_too_many_args, argv[0], NULL);
           }
           parmp->had_stdin_file = true;
           parmp->edit_type = EDIT_STDIN;
@@ -1137,7 +1138,7 @@ static void command_line_scan(mparm_T *parmp)
           nlua_disable_preload = true;
         } else {
           if (argv[0][argv_idx]) {
-            mainerr(err_opt_unknown, argv[0]);
+            mainerr(err_opt_unknown, argv[0], NULL);
           }
           had_minmin = true;
         }
@@ -1211,7 +1212,7 @@ static void command_line_scan(mparm_T *parmp)
         break;
       case 'q':    // "-q" QuickFix mode
         if (parmp->edit_type != EDIT_NONE) {
-          mainerr(err_too_many_args, argv[0]);
+          mainerr(err_too_many_args, argv[0], NULL);
         }
         parmp->edit_type = EDIT_QF;
         if (argv[0][argv_idx]) {  // "-q{errorfile}"
@@ -1240,7 +1241,7 @@ static void command_line_scan(mparm_T *parmp)
         break;
       case 't':    // "-t {tag}" or "-t{tag}" jump to tag
         if (parmp->edit_type != EDIT_NONE) {
-          mainerr(err_too_many_args, argv[0]);
+          mainerr(err_too_many_args, argv[0], NULL);
         }
         parmp->edit_type = EDIT_TAG;
         if (argv[0][argv_idx]) {  // "-t{tag}"
@@ -1274,7 +1275,7 @@ static void command_line_scan(mparm_T *parmp)
       case 'c':    // "-c{command}" or "-c {command}" exec command
         if (argv[0][argv_idx] != NUL) {
           if (parmp->n_commands >= MAX_ARG_CMDS) {
-            mainerr(err_extra_cmd, NULL);
+            mainerr(err_extra_cmd, NULL, NULL);
           }
           parmp->commands[parmp->n_commands++] = argv[0] + argv_idx;
           argv_idx = -1;
@@ -1291,19 +1292,19 @@ static void command_line_scan(mparm_T *parmp)
         break;
 
       default:
-        mainerr(err_opt_unknown, argv[0]);
+        mainerr(err_opt_unknown, argv[0], NULL);
       }
 
       // Handle option arguments with argument.
       if (want_argument) {
         // Check for garbage immediately after the option letter.
         if (argv[0][argv_idx] != NUL) {
-          mainerr(err_opt_garbage, argv[0]);
+          mainerr(err_opt_garbage, argv[0], NULL);
         }
 
         argc--;
         if (argc < 1 && c != 'S') {  // -S has an optional argument
-          mainerr(err_arg_missing, argv[0]);
+          mainerr(err_arg_missing, argv[0], NULL);
         }
         argv++;
         argv_idx = -1;
@@ -1312,7 +1313,7 @@ static void command_line_scan(mparm_T *parmp)
         case 'c':    // "-c {command}" execute command
         case 'S':    // "-S {file}" execute Vim script
           if (parmp->n_commands >= MAX_ARG_CMDS) {
-            mainerr(err_extra_cmd, NULL);
+            mainerr(err_extra_cmd, NULL, NULL);
           }
           if (c == 'S') {
             char *a;
@@ -1343,7 +1344,7 @@ static void command_line_scan(mparm_T *parmp)
           if (strequal(argv[-1], "--cmd")) {
             // "--cmd {command}" execute command
             if (parmp->n_pre_commands >= MAX_ARG_CMDS) {
-              mainerr(err_extra_cmd, NULL);
+              mainerr(err_extra_cmd, NULL, NULL);
             }
             parmp->pre_commands[parmp->n_pre_commands++] = argv[0];
           } else if (strequal(argv[-1], "--listen")) {
@@ -1425,7 +1426,7 @@ scripterror:
 
       // Check for only one type of editing.
       if (parmp->edit_type > EDIT_STDIN) {
-        mainerr(err_too_many_args, argv[0]);
+        mainerr(err_too_many_args, argv[0], NULL);
       }
       parmp->edit_type = EDIT_FILE;
 
@@ -1472,7 +1473,7 @@ scripterror:
   }
 
   if (embedded_mode && (silent_mode || parmp->luaf)) {
-    mainerr(_("--embed conflicts with -es/-Es/-l"), NULL);
+    mainerr(_("--embed conflicts with -es/-Es/-l"), NULL, NULL);
   }
 
   // If there is a "+123" or "-c" command, set v:swapcommand to the first one.
@@ -2135,28 +2136,30 @@ static int execute_env(char *env)
   return OK;
 }
 
-/// Prints the following then exits:
-/// - An error message `errstr`
-/// - A string `str` if not null
+/// Prints a message of the form "{msg1}: {msg2}: {msg3}", then exits with code 1.
 ///
-/// @param errstr  string containing an error message
-/// @param str     string to append to the primary error message, or NULL
-static void mainerr(const char *errstr, const char *str)
+/// @param msg1  error message
+/// @param msg2  extra message, or NULL
+/// @param msg3  extra message, or NULL
+static void mainerr(const char *msg1, const char *msg2, const char *msg3)
   FUNC_ATTR_NORETURN
 {
-  print_mainerr(errstr, str);
+  print_mainerr(msg1, msg2, msg3);
   os_exit(1);
 }
 
-static void print_mainerr(const char *errstr, const char *str)
+static void print_mainerr(const char *msg1, const char *msg2, const char *msg3)
 {
   char *prgname = path_tail(argv0);
 
   signal_stop();              // kill us with CTRL-C here, if you like
 
-  fprintf(stderr, "%s: %s", prgname, _(errstr));
-  if (str != NULL) {
-    fprintf(stderr, ": \"%s\"", str);
+  fprintf(stderr, "%s: %s", prgname, _(msg1));
+  if (msg2 != NULL) {
+    fprintf(stderr, ": \"%s\"", msg2);
+  }
+  if (msg3 != NULL) {
+    fprintf(stderr, ": \"%s\"", msg3);
   }
   fprintf(stderr, _("\nMore info with \""));
   fprintf(stderr, "%s -h\"\n", prgname);