correctly interpret -1 as unlimited in invoice

also:
- fix improper pluralization in a translation
- fix sign on excess sms's
I dunno, these are things that should logically never happen. I think of named exceptions as being for specific known cases. Anyhow, I think in this case it doesn't affect too much, since `Exception` is caught right below it. I'll offline with you; if you guys already have a system, I should go with that.