internal/lsp: log errors when diagnostics fail
internal/lsp: log errors when diagnostics fail

This will allow us to better track the errors that might cause golang/go#30786.

Change-Id: I310ad0fd0eda3102d5763e28fc383a4994ef0ff3
Reviewed-on: https://go-review.googlesource.com/c/tools/+/172280
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
Reviewed-by: Ian Cottrell <52376736e5e84615ec49f9c1843b8acc178b1d8d@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools