don't use the kerberos.GSSError.message attribute

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.

sleekxmpp/util/sasl/mechanisms.py

@@ -541,7 +541,7 @@ else:
 
                 resp = kerberos.authGSSClientResponse(self.gss)
             except kerberos.GSSError as e:
-                raise SASLCancelled('Kerberos error: %s' % e.message)
+                raise SASLCancelled('Kerberos error: %s' % e)
             if not resp:
                 return b''
             else: