Error-handling for principals that don't parse.
Add convenient methods (and error code) to get enctypes.

Trying to get rid of this Err.Context.KEY thing.
Add a new krb.ProtocolError exception.
Add krb.KeyTypeMismatchError.

Pffft. Map it to a KDC.ProtocolError since it would be a protocol error
in that case.
No more Err.Context.
Rethrow protocol errors.
Cross-realm not supported is not an internal error.

Well, it is but meh. :-/
Rebuild webathena.js