alert when code action fails

[justschen]

          Yeah I'm inclined to say that we should surface this error to the user so they can report it. Ia actually thought that's what we were doing on the VS Code side by calling `onUnexpectedExternalError`:

vscode/src/vs/editor/contrib/codeAction/common/types.ts

Line 185 in cb807ec

onUnexpectedExternalError(err);

But it turns out we end up silently catching the error

@justschen Want to try changing our code to alert when resolve fails?

Originally posted by @mjbvz in microsoft/language-server-protocol#2112 (comment)