PYCBC-116: Clarify error messages

These will hopefully provide clearer error messages in the case of
common errors.

Change-Id: Icead68ef9a5a2e9244f3bd695e3420b155ed67f8
Reviewed-on: http://review.couchbase.org/26371
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>