Clarify description of ServiceNotAuthorizedException

Clarify description of ServiceNotAuthorizedException

This exception's detail message states

	Service not permitted

and according to the Javadoc it indicates that the current user does not
have access to the service.  In practice, though, callers handle this
exception by presenting a '401 Unauthorized' response to the client,
meaning that the user is unauthenticated and should authenticate.

Clarify the documentation and detail message to match the practice.

The exception message is not used anywhere except logs.  No
client-visible effect intended.

Change-Id: I2c6be9cb74af932f0dcb121a381a64f2ad876766
Signed-off-by: Jonathan Nieder <jrn@google.com>