Fixes for Python 3, Django 1.7

1. The recent workaround for handling get_user_model() at load
time in settings.py was nearly sufficient, but in Django 1.7
the incompletely-loaded app registry throws a RuntimeError,
not an ImportError. The relevant except-statement in settings.py
has been extended to accept RuntimeError as well.

2. BaseException.message was deprecated for Python 3. In views
handling StripeErrors, some code accesses e.message, which causes
AttributeErrors from its nonexistence. A few of these usages have
been replaced with str(e), which seems to produce suitable output.
Several usages of e.message remain, and will likely require fixing
later. Better testing of str(e) with the routines using e.message
will be required.