Rethrow exception

If a ShareJS auth function throws an exception,
the following stack is present:

- auth function
- useragent
- session
- EventEmitter
- BC server
- ...

By not re-throwing the exception at this point any exceptions
thrown between the session message event handler and the ShareJS
auth function are swallowed at this point.
Try-wrap JSON parsing only, close session on error
Return in catch if JSON parsing failed
Merge pull request #19 from evilhackerdude/patch-1

Rethrow exception