Rework the invalid argument for init sec context exception

Rework the invalid argument for init sec context exception

* GSSException:
  * Separate out messages for major an minor error messages
  * Reduce the what() message
  * Change the flow so that major and minor messages are calculated at initialization time, so that everything else can be guaranteed not to throw any exceptions
* GSSRequest:
  * Standardize the formatting of the errors returned by a GSSException
* GSSInitSecContext
  * Throw GSSException instead of invalid_argument when the mech is unacceptable