Add check Identity validate when get schemas

Glance client don't check Identity validate when get schemas.
So when you exec "glance image-create" with invalid credentials in
glance-api.conf, it returns "unrecognized arguments: --foo".
It is difficult to debug such message.

This change makes invalid credentials error clear.

Change-Id: Ib641333cd8d51f459df70306a1eeda250ada5ca1
Closes-Bug: 1467719

glanceclient/shell.py

@@ -577,6 +577,9 @@ class OpenStackImagesShell(object):
 
                     with open(schema_file_path, 'w') as f:
                         f.write(json.dumps(schema.raw()))
+                except exc.Unauthorized:
+                    raise exc.CommandError(
+                        "Invalid OpenStack Identity credentials.")
                 except Exception:
                     # NOTE(esheffield) do nothing here, we'll get a message
                     # later if the schema is missing