Instead of just underlining the first char of an identifier, we now underline. the full identifier.
Still haven't used them in the Vim client though, but will soon.
Previously we'd show a Python traceback if the user asked for a detailed diagnostic in a file that wasn't supported by Clang (something written in Python perhaps). Now we show an nice, far less scary message informing the user of this. Fixes #748.
Fixes #216
