Store the results of all worker processes

[dethos]

There should be a log of failed worker tasks (such as emails that could not be sent) in order to be able to retry later or inform the user that a message could not be delivered.

Also, but not a priority, a log of successful tasks would be valuable, in order to have some kind of prof that the messages left the system correctly.

[dethos]

One good option for this issue would be to integrate Flower on the project.