Print human friendly string as an error message

This commit also removes a blank line displayed
when error_dict['detail'] is empty.

When 403 is returned, Forbidden exception is raised from
HTTPClient (neutronclient/client.py). There is no reason
this exception is raised by it. By this commit v2_0.client
now handles 403 response to show human friendly message
by parsing Neutron 2.0 -specific error format.

Change-Id: Iefbd7db094267fe3799ebf4713e142e62c63174c
Closes-Bug: #1246271
Merge "Print human friendly string as an error message"