Catch a peer API workflow exception

We should have been catching `PeerAssessmentWorkflowError` in the peer assessment mixin, but weren't.  The error occurs when someone tries to assess a peer without first obtaining a lease on the submission.  We've seen this error a few times in the logs and downgraded it to a warning, but because we weren't catching it, it was causing a 500 error.

@stephensanchez