Manager._receive_data: split condition for `_message_queue.put`

The program logic is the same but we get more precise log messages.
Manager: add attr `errors_in_threads` (queue)
Manager: put msg in `errors_in_threads` when there are errors

We do it after putting the "sentinel object" in the corresponding queue.
The msg is the same one we log (with level "warning" or "error").
Manager: add docstring with doc of `errors_in_threads`
Merge pull request #9 from medularis/feature-manager-errors_queue

Manager: queue for errors in threads