Catch the ActiveRecord::RecordNotFound and trigger an unsubscribe

[collimarco]

If for some reasons (e.g. lost connection) the feed gets deleted from the database and the subscription is not properly removed from Superfeedr, it would be useful to catch the ActiveRecord::RecordNotFound exception (raised here) and rescue with a SuperfeedrEngine::Engine.unsubscribe(@feed).
This is safe since that exception is only raised if the database query is successful and the record is not found.

[collimarco]

Moreover the check on the line after find is wrong. find doesn't return nil, it raises an exception!