Check for err before checking the response code

In the case where we have an error and no response object, we crash on:

TypeError: Cannot read property 'statusCode' of undefined

We should be able to assume that we have either an error or a response
object, so just switching the order in the conditional should prevent
checking the null response object.
Assert request-level errors are passed through.

If an error is made when making a request (say, internet connection is disabled), that error should be returned through node-trello.
Merge branch 'master' of github.com:brettkiefer/node-trello into fix-err-handling
Publish v1.0.1.

Includes fix for request-level errors throwing `TypeError: Cannot read property 'statusCode' of undefined`. (#15).