Fixing typo

Previously, statusCode is undefined.
Error code handling

According to 500px docs:

> Return values are 200 if everything went fine. If the response code is 403, the resulting JSON contains an error key/value pair with a description of the problem. A common error value is "The photo specified is already in your favorites."

It's generally better to return error message from 500px as well as otherwise, finding bugs is almost impossible.
Merge pull request #10 from grabbou/patch-1

Better error handling