Log errors from the routine polling producers.

Currently, they just go silently into the log file.  Furthermore, they
currently *kill* the producer thread, so if one batched email fails, they all
*stop* until one of us restarts the fedmsg-hub daemon.

This change 1) logs errors and more importantly 2) leaves the producer running.
Try to get encoding right with email messages.

This works against the current bug that had the production instance backed up.
Use to_bytes.
The unicode sandwich is king.
Merge pull request #31 from fedora-infra/feature/producer-errors

a00e51c - Log errors from the routine polling producers.