backport: fix(lsp): Ensure human readable errors are printed

`return err_message(tostring(err))` caused errors to be printed as
`table: 0x123456789` instead of showing the error code and error
message.

This also removes some `if err` blocks that never got called because at
the end of `handlers.lua` all the handlers are wrapped with logic that
adds generic error handling.

runtime/lua/vim/lsp/handlers.lua

@@ -18,10 +18,8 @@ local function err_message(...)
 end
 
 --@see https://microsoft.github.io/language-server-protocol/specifications/specification-current/#workspace_executeCommand
-M['workspace/executeCommand'] = function(err, _)
-  if err then
-    error("Could not execute code action: "..err.message)
-  end
+M['workspace/executeCommand'] = function()
+  -- Error handling is done implicitly by wrapping all handlers; see end of this file
 end
 
 -- @msg of type ProgressParams
@@ -158,13 +156,12 @@ M['workspace/applyEdit'] = function(_, _, workspace_edit)
 end
 
 --@see https://microsoft.github.io/language-server-protocol/specifications/specification-current/#workspace_configuration
-M['workspace/configuration'] = function(err, _, params, client_id)
+M['workspace/configuration'] = function(_, _, params, client_id)
   local client = vim.lsp.get_client_by_id(client_id)
   if not client then
     err_message("LSP[id=", client_id, "] client has shut down after sending the message")
     return
   end
-  if err then error(vim.inspect(err)) end
   if not params.items then
     return {}
   end
@@ -199,11 +196,7 @@ end
 --@param map_result function `((resp, bufnr) -> list)` to convert the response
 --@param entity name of the resource used in a `not found` error message
 local function response_to_qflist(map_result, entity)
-  return function(err, _, result, _, bufnr)
-    if err then
-      vim.notify(err.message, vim.log.levels.ERROR)
-      return
-    end
+  return function(_, _, result, _, bufnr)
     if not result or vim.tbl_isempty(result) then
       vim.notify('No ' .. entity .. ' found')
     else
@@ -443,7 +436,12 @@ for k, fn in pairs(M) do
     })
 
     if err then
-      return err_message(tostring(err))
+      -- LSP spec:
+      -- interface ResponseError:
+      --  code: integer;
+      --  message: string;
+      --  data?: string | number | boolean | array | object | null;
+      return err_message(tostring(err.code) .. ': ' .. err.message)
     end
 
     return fn(err, method, params, client_id, bufnr, config)