Document error handling

[mjallday]

https://botbot.me/freenode/balanced/2014-09-10/?msg=21410510&page=2

We do not have any clear examples of how best to handle errors in the various clients.

I looked at the client libraries themselves as well as the rentmybike example applications. The example apps catch generic exception types, they don't even bother to catch specific errors.

Can we add examples of how to handle a couple of different types or errors (insufficient-funds, card-declined and general validation error would be a good start)

[trenton42]

It would also be nice to see the category_code list grouped by status_code in the docs as well as the possible three digit codes that prefix the description field for things like card-declined (I just looked at 5 different card-declined errors in our logs and saw 5 different descriptions).