Add a specific type to represent errors (implementing error interface, of course)

[ChimeraCoder]

Right now error handling is pretty basic, but a dedicated error type would help applications that want to take different actions based on the specific error received (ie, to slow down when a 'Rate limit exceeded' error is encountered, versus a 'Sorry, that page does not exist" error.

(Even when we implement rate limiting as in #1, this will still be needed because we can't be sure that the same credentials aren't being used by some other application simultaneously).