Show exceptions in text format for xhr requests

For developers who are debugging their code in web inspector, it would be great to see exceptions in text format:

Small preview:

![screen shot 2013-08-21 at 4 43 47 pm](https://f.cloud.github.com/assets/522155/1001047/5818f780-0a5f-11e3-869b-75ac4b502201.png)