Change appr error if you try to push to an image repository

501 was confusing to the users, so change to 405 and add an error explaining why the push failed

Fixes https://github.com/app-registry/helm-plugin/issues/8#issuecomment-291553739

endpoints/appr/test/test_appr_decorators.py

@@ -1,6 +1,6 @@
 import pytest
 
-from werkzeug.exceptions import NotImplemented as NIE
+from werkzeug.exceptions import HTTPException
 
 from data import model
 from endpoints.appr import require_app_repo_read
@@ -14,6 +14,6 @@ def test_require_app_repo_read(app):
   def empty(**kwargs):
     called[0] = True
 
-  with pytest.raises(NIE):
+  with pytest.raises(HTTPException):
     empty(namespace='devtable', package_name='simple')
     assert not called[0]