codelens: go mod tidy or some gopls provided codelenses can fail with cryptic error popups

[hyangah]

Some commands run by gopls require all changes in buffer to be saved. Otherwise they return errors.
The error details are not currently visible unless users inspect the gopls (server) output channel.
Instead vscode will pop up rather cryptic error messages.

This is related to the upstream bug reports microsoft/vscode#116932 and microsoft/vscode#116953

Here is the LSP trace.

[Trace - 09:15:50.467 AM] Sending request 'workspace/executeCommand - (124)'.
Params: {"command":"gopls.tidy","arguments":[{"URIs":["file:///Users/hakim/projects/google/w/go.mod"]}]}

[Error - Received] 09:15:50.468 AM #124 All files must be saved first

[Error - 9:15:50 AM] Request workspace/executeCommand failed.
  Message: All files must be saved first
  Code: 0 

Would be nice if VS Code & vscode-languageclient come up with a solution to surface the error.
If not, some workarounds I can think of are:

• From middleware, check if all the files are saved and either force to save or show popup for user to save it, or
• From middleware, surface the error message using a popup and swallow the error (not sure if it's possible), or
• Let gopls return a popup rather than reporting this as an error.

cc @stamblerre

[gopherbot]

Change https://golang.org/cl/300389 mentions this issue: src/goLanguageServer: show command failure error messages from gopls

[hyangah]

After:

Before: