Ignore non-"delete" SQS events, rather than logging an error.

As noted in #71, the auth-server will soon start sending other event types on the SQS message channel.  This PR patches our event-processing script to ignore them rather than log error tracebacks about them.

There's a *slight* change in semantics here in that any `(ValueError, KeyError)` raised by `backend.retire_user` will be reported with a different log message.  It shouldn't ever raise such though, so I think this approach wins in terms of code clarity.

@fmarier r?