Raise a new exception NotImplemented for HTTP501

In _error_checker(), both HTTP500 and 501 are converted to the same
exception ServerFault. In addition, some method which extracts error
message raises a specific exception IdentityError without considering
HTTP code.
This patch adds a new exception NotImplemented and uses it for HTTP501
so that we can know which error response is returned from a server.

Change-Id: Ic8fc216377942619f11a2462b79d0597071ac294
Merge "Raise a new exception NotImplemented for HTTP501"