Added optional callback SqlalchemyDataLayer.unhandled_exception_forma…

[tadams42]

Related to #72

I'd introduced another optional argument for SQLAlchemy data layer that works like this:

def sa_exception_formatter(exc):
    return {"detail": "SQLAlchemy complains!", "status": 405}

class ComputerList(ResourceList):
    schema = ComputerSchema
    data_layer = {'session': db.session,
                  'model': Computer,
                  'unhandled_exception_formatter': sa_exception_formatter}

I was playing with idea to generalize this more, but after inspecting existing code, I don't think it is needed - in 99% cases flask-rest-jsonapi already does the only sane thing.

So, this new callback gets (optionally) called only in places where unexpected exception had happened.

What do you think?

[coveralls]

Coverage increased (+0.08%) to 91.408% when pulling a8e5e3f on tadams42:custom_sa_data_layer_exceptions into ecc8f2c on miLibris:master.

[coveralls]

Coverage increased (+0.08%) to 91.408% when pulling a8e5e3f on tadams42:custom_sa_data_layer_exceptions into ecc8f2c on miLibris:master.

[coveralls]

Coverage increased (+0.09%) to 91.132% when pulling 72e978c on tadams42:custom_sa_data_layer_exceptions into b4cb557 on miLibris:master.