Improve error for token issue command without auth

Currently when you perform the 'token issue' command with
the admin_token, the error is a nonsense python error. This
commit changes it to be user-friendly.

Change-Id: I5cc92c342e3f83e099354cd04301c7b8d8d2dabc
Closes-Bug: #1547721

Author: timothy-symanczyk

openstackclient/identity/v3/token.py

@@ -18,6 +18,7 @@
 import six
 
 from openstackclient.common import command
+from openstackclient.common import exceptions
 from openstackclient.common import utils
 from openstackclient.identity import common
 
@@ -172,6 +173,9 @@ def get_parser(self, prog_name):
         return parser
 
     def take_action(self, parsed_args):
+        if not self.app.client_manager.auth_ref:
+            raise exceptions.AuthorizationFailure(
+                "Only an authorized user may issue a new token.")
         token = self.app.client_manager.auth_ref.service_catalog.get_token()
         if 'tenant_id' in token:
             token['project_id'] = token.pop('tenant_id')