Here's two failing tests for some payday bugs

These address regressions introduced by #2036.
Handle the Balanced API more saferly during payday

This makes us more robust in the face of failures communicating with the
Balanced API. Now we've got a blanket try/except around the parts where
we talk to Balanced, so we will never totally crash because of a
mishandling of some sort. The ach_credit side is not as well-factored
as the charge_on_balanced side, and is not tested as a result. This
commit nonetheless attempts to implement the same error handling on both
sides.
Take out probably misguided check for merchants

At https://github.com/gittip/www.gittip.com/pull/2036/files#r9757101
@mjallday suggests that we don't need to check that merchants are
underwritten, as that constraint now only applies to those receiving >
$100,000/yr, and we don't have anyone there yet. Moreover, I believe
this condition is backwards, and would result in no-one getting paid!
Without any kind of condition here, I suspect that we'll fill our logs
with NoResultError()s for everyone who is due a payout and has a CC on
file with Balanced but not a bank account. Better that than not doing
any payouts. :-)
Merge pull request #2064 from gittip/fix-payday-regressions

Fix payday regressions