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
New exception when auth_url is not specified

Certain scenarios into the neutron client will not specify the
auth_url.  This is typically when a token is specified.  However, when
the token is expired the neutron client will attempt to refresh the
token.  Users of this may not have passed in all of the required
information for this reauthentication to properly occur.

This code fixes an error that occurs in this flow where the auth_url
(which is None) is appended to another string.  This results in a core
Python error.

The update will provide a more targetted error message specifying to
the user that the auth_url needs to be specified.  An associated unit
test is also included to validate this behavior.

Change-Id: I577ce0c009a9a281acdc238d290a22c5e561ff82
Closes-Bug: #1241275
Updated from global requirements

Change-Id: I24869d188b7ecdc2a08ed10e1aafdeee2914d127
Merge "Print human friendly string as an error message"
Merge "Updated from global requirements"
Merge "New exception when auth_url is not specified"