List NotImplementedError as a client exception for vnc

Not all drivers implement the get_vnc_console method.  In that case,
they will raise NotImplementedError.  List this exception as a client
exception so that the rpc layer does not log a traceback when this
happens.

Change-Id: Id30fd35b36cbd222b2d8787f00fa586d1ed4f11a
Merge "List NotImplementedError as a client exception for vnc"