Fixes for assignments crashing on invalid signups

https://code.google.com/p/otwarchive/issues/detail?id=3815
Update challenge_giftexchange.feature

Argh, tweaked the warning text and forgot to edit the test. :P
Fixing signup -> sign-up in user-facing text
Signups -> sign-ups in mailer subject line
Signups -> sign-ups in message content
Signups -> sign-ups in message content
Signup -> sign-up in test
Signup -> sign-up in help file
Merge pull request #1567 from shalott/issue_3815_assignments_hanging

Fixes for assignments crashing on invalid signups (issue 3815)