Register error handlers

[Everett Toews]

Add the capability to allow users to register error handlers.

Users would be able to register error handlers. These handlers would be invoked when an errors occurs around the code that makes the request to the API. The handlers would receive the full context (full request and response) as params to a handler function call.

The user could do whatever they need to with the error. Including re-raising the error after some processing.

[Ian Cordasco (icordasc)]

Why not include the response object on the final exception? That has the original request object attached.