more readable error output from interpretXhrFail

I'm not sure if I'm missing something, but I *think* this change should always make the output of `interpretXhrFail` more readable/informative for legitimate HTTP errors. Basically it changes this:

![screen shot 2014-08-05 at 9 03 53 am](https://cloud.githubusercontent.com/assets/409328/3814322/5f1b1446-1cba-11e4-8354-e342e30ee436.png)

To this:

![screen shot 2014-08-05 at 9 06 39 am](https://cloud.githubusercontent.com/assets/409328/3814340/82dfeb86-1cba-11e4-81ca-0233f04d03d4.png)