Mail Handler error handling improvement

If an email cannot be fully processed, the handler retries importing the email again in the next iteration. It may result in many identical issues being created.

It is also possible, that the email is processed, but the issue cannot be saved, an exception is thrown and the email remains in the queue to be picked up in the next handler iteration resulting in huge gaps between the issue keys.

Solution: handle errors better and mark emails not processable blocking it from being processed again.