Allow finer control of API errors in client code (issue #43)

[eggpi]

How about something like this for #43? This also includes a little bit of cleanup, and I think it's ready for a first look.

I've only tested manually so far (basically following the examples in the README with different retry parameters). I think I might be able to add some unit tests later, but let me know if there are any other testing you normally do.