Use message from ServiceNotAuthorizedException, ServiceNotEnabledException

Use message from ServiceNotAuthorizedException, ServiceNotEnabledException

When sending an error response due to ServiceNotAuthorizedException or
ServiceNotEnabledException, usually we send a default message.  In the
ServiceNotEnabledException case, we use

	403 Git access forbidden

except in a dumb-HTTP-specific filter where we use the servlet
container's default 403 response:

	403 Forbidden

In the ServiceNotAuthorizedException case, we use the servlet
container's default 401 response:

	401 Unauthorized

There is one exception: a ServiceNotEnabledException when handling a
smart HTTP /info/refs request uses the message from the exception:

	403 Service not enabled

Be more consistent by always using the message from the exception.  This
way, authors of a RepositoryResolver, UploadPackFactory, or
ReceivePackFactory can provide a more detailed message when appropriate.
The defaults are

	401 Unauthorized
	403 Service not enabled

Change-Id: Id1fe1c2042fb96487c3671c1965c8a65c4b8e1b8
Signed-off-by: Jonathan Nieder <jrn@google.com>