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)
+

endpoints/v2/v2auth.py

@@ -6,12 +6,12 @@ from flask import request, jsonify, abort
 
 from app import app, userevents, instance_keys
 from auth.auth_context import get_authenticated_user, get_validated_token, get_validated_oauth_token
-from auth.decorators import process_auth
+from auth.decorators import process_basic_auth
 from auth.permissions import (ModifyRepositoryPermission, ReadRepositoryPermission,
                               CreateRepositoryPermission, AdministerRepositoryPermission)
-from auth.process import process_auth
-from endpoints.v2 import v2_bp
 from endpoints.decorators import anon_protect
+from endpoints.v2 import v2_bp
+from endpoints.v2.errors import InvalidLogin
 from data.interfaces.v2 import pre_oci_model as model
 from util.cache import no_cache
 from util.names import parse_namespace_repository, REPOSITORY_NAME_REGEX
@@ -34,10 +34,10 @@ def get_scope_regex():
 
 
 @v2_bp.route('/auth')
-@process_auth
+@process_basic_auth
 @no_cache
 @anon_protect
-def generate_registry_jwt():
+def generate_registry_jwt(auth_result):
   """
   This endpoint will generate a JWT conforming to the Docker Registry v2 Auth Spec:
   https://docs.docker.com/registry/spec/auth/token/
@@ -57,11 +57,11 @@ def generate_registry_jwt():
   oauthtoken = get_validated_oauth_token()
   logger.debug('Authenticated OAuth token: %s', oauthtoken)
 
-  auth_credentials_sent = bool(request.headers.get('authorization', ''))
+  auth_header = request.headers.get('authorization', '')
+  auth_credentials_sent = bool(auth_header)
   if auth_credentials_sent and not user and not token:
     # The auth credentials sent for the user are invalid.
-    logger.debug('Invalid auth credentials')
-    abort(401)
+    raise InvalidLogin(auth_result.error_message)
 
   access = []
   user_event_data = {