Code intel: show an error notification when the language server is misconfigured

[chrismwendt]

It's difficult to get the global settings for a language server correct. It would be helpful to have some proactive error notifications to help during setup.

I started working on this in sourcegraph/sourcegraph-go#79. It'll probably get merged into lsp-client at some point.

[efritz]

We're no longer putting effort into language server support.