When the GSSAPI mechanism's process() function is invoked for the third time (on success) it must not attempt further processing. Instead it should clean the context and return an empty response.
Replaced the reference to kerberos.GSSError.message in any raised exception, because:
DeprecationWarning: BaseException.message has been deprecated as of Python 2.6
and its natural repr is probably the most desirable output.
