[clangd] Don't send invalid messages from remote index

Summary:
Remote server should not send messages that are invalid and will cause problems
on the client side. The client should not be affected by server's failures
whenever possible.

Also add more error messages and logs.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83826