PYCBC-253: Attempt to handle non-CouchbaseError exceptions

If caught during a callback, errors such as MemoryError should be passed
back to the user, rather than crashing the application.

Change-Id: I316892d68673125a636d442eea42a63eb80a1bff
Reviewed-on: http://review.couchbase.org/39486
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>