Test the promise being rejected due to an error

[These](https://gist.github.com/jmeas/2aefd32c4032293e39b5) are (roughly) the tests.

I was unable to do this because mocha flips out when the `fetch` method throws an error. If anyone knows a way to test [this code path](https://github.com/jmeas/backbone.object-router/blob/d056160b96d44e80fdb69b575e02c08efb226ffb/src/router.js#L49-L52) – let me know!

Start with src/router.js lines 49-52 and compare the expected cases in the linked gist. Reproduce the Mocha failure when fetch throws, then add a test covering the rejected promise; done means the error path is exercised without Mocha failing unexpectedly.

Written by the indexing model from the issue text.