Return more helpful errors when unrecognized data is fetched

[aaronpk]

Some recent troubleshooting has led people down the path of checking their markup, but their markup is fine when pasted into the debug forms in XRay. The failure instead has to do with XRay receiving an unexpected result when fetching the content. Currently XRay shows the http status code in the response, but it would be helpful to include more clues to indicate the fetch is the problem rather than the markup. Some ideas: number of bytes received, content type header received.