Signal invalid subscriptions via the API

Currently invalid subscriptions fails silently and a parsing Task is kept in the database without any Subscription attached to it. There's no simple way for the ML user to know if a subscription parsing Task has failed.

One solution for that might be to:

• Create a specific callback for failed subscription update and creation.
• Either delete Tasks when creation failed and try count is at 0 or provide the user a way to clear the task.

One other way could be to create a Subscription in any cases and add a field Status that reflects parse or refresh failures to the ML user. The ML user can then decide to remove the Subscription via the API which would lead to the Task removal.