mpd.py: catch all exceptions raised by _hello()

mpd.py: catch all exceptions raised by _hello()

I normally consider it bad form to catch all exceptions, since it can
result in lost KeyboardInterrupts.  But in this case we re-raise the
exception after cleaning up, so it's ok.

While _hello() itself only raises socket.error or an MPDError subclass, a
KeyboardInterrupt would previously go uncaught.  Since the connection was
never cleaned up, it could be left in an unusable state.  So just
disconnect on all exceptions instead.