support quickfix mode about tern

[marijnh]

Infer does indeed not (and, I think, never will) produce type errors. That's just not something that makes sense given the algorithm used.

If someone wants to do the work involved in making acorn_loose.js track errors, I'll gladly take a look at it, but that would considerably complicate the code, and cause some issues like making it harder to parse file fragments (and to report meaningful error sets when a fragment was sent to the server).

For now, this is out of scope, and I am not going to work on it myself.