Fixes for assignments crashing on invalid signups (issue 3815)

RELEASE NOTES: Invalid signups can sometimes be created which subsequently cause assignments to crash silently in the background. We now alert individual users if their signup is invalid, and also check for invalid signups and notify the moderators by email before running potential match generation, so they can fix them before they get to the problematic step. 

https://code.google.com/p/otwarchive/issues/detail?id=3815