allow negative balance
Merge branch 'master' into negative-balance
Use a custom exception instead of overloading

We were overloading psycopg2.IntegrityError to model the case where
balance shouldn't be negative. Let's use a custom exception instead to
avoid muddying the waters.
Failing test for negative balance from rec-an-exch
Fix record-an-exchange to allow negative balance
Upgrade balanced dev marketplace

I was hitting stale VCR test fixtures and ended up upgrading the
Balanced marketplace account we're using in dev amidst my flailing.
Test fail for negative balance during exchange
Add check for negative balance in exchanges
Merge pull request #2434 from gittip/negative-balance

Allow negative balance