always show response headers and show error bodies

I made a couple changes to the inspector that I hope you'll find useful.

1. Always show the response headers.  They were shown for failed requests before, but often I wanted to see them for successful requests too (without needing to poke around in the developer tools).
2. Show response body text for failed ajax requests.  If the body is json, pretty print it.  (Handy for seeing validation errors in response to POST or PUT, for example.)

As an example of these changes in action, check out http://audreydemo.aws.af.cm/hal-browser/#/people
From there, do a POST request on "self" (leaving the dialog defaults alone).  You should see that the response body is a small json document with an error message.  With the current version of HAL-Browser, the response body wouldn't be shown.  (By the way, if you check out the demo, I'd love to hear your thoughts on my use of HAL.)