[clangd][vscode] Surface the error when applying tweaks fails

Summary:
The current behavior for a failed request is just to log it in the
output panel. When applyTweak fails for whatever reason, users usually don't get
informed (unless they open the output panel and dig the log).

this patch is to surface these errors by prompting up a message diag.

Reviewers: ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66211

llvm-svn: 368851