A few weeks back a new error type `idempotency_error` was introduced in
the API. I put it in to respond to #503, but then forgot to add support
for it in this library. This patch introduces a new exception class that
represents it.
Right now we have every error class in a separate file which doesn't do
anyone a lot of good, especially given that most of them are just stubs
that derive directly from `StripeError`.
This patch pulls them all into one file and gives them some
documentation. It follows #487 and #488 as minor refactoring work.
