API request limits

[Andrew-Woodford]

There is a very strict limit on the number of requests that can be performed sequentially (2 per app per second). This means if you want to do several operations in quick succession (i.e. generate something, see if it's been successful then access it) you are likely to hit this upper bound which will result in an insufficient permissions error.

It would be useful to have:

1. A distinct error type when API request limits have been hit.
2. Wrapping the requests interface to include some kind of 'last requests time' which imposes this limit automatically.