Proper error reporting

[zxol]

Currently, the library does not have adequate error reporting.

When I initially wrote this library, it was part of a project that used a simple null check to essentially ignore errors. Quite daft, I know. :/

So I think I will need to make some serious changes to the API. I will change it to return a rejected promise with a JS Error object. It will contain either a wrapper level error (i.e. a missing argument) or a error response from airbnb (for example, "listing not found", or "you must be authorized to do that")