Make V2 login errors more descriptive

If login fails, we now call validate again to get the reason for the failure, and then surface it to the user of the CLI. This allows for more actionable responses, such as:

$ docker login 10.0.2.2:5000
Username (devtable): devtable
Password:

Error response from daemon: Get http://10.0.2.2:5000/v2/: unauthorized: Client login with unencrypted passwords is disabled. Please generate an encrypted password in the user admin panel for use here.

endpoints/v2/errors.py

@@ -146,3 +146,12 @@ class Unsupported(V2RegistryException):
                                       'The operation is unsupported.',
                                       detail,
                                       405)
+
+
+class InvalidLogin(V2RegistryException):
+  def __init__(self, message=None):
+    super(InvalidLogin, self).__init__('UNAUTHORIZED',
+                                       message or 'Specified credentials are invalid',
+                                       {},
+                                       401)
+