Pass through error clues in Deferred

[Adam Peller]

I have to confess, I'm a dojo.xhr n00b. My first attempt at using this in quite some time led to a "Deferred Cancelled" error message. I did the lazy thing to just define an error handler that spit out the response object, and "Deferred Cancelled" was pretty meaningless. Instead, I had to go digging for this information as cancelResult.

It would seem more informative, at least in the example I had (file not found) to pass in res or res or perhaps res or res.toString()

"Deferred Cancelled" when wrapping the Error, rather than just the string "Deferred Cancelled".

[James Burke]

(In [16050]) Fixes #8023, allow for more descriptive cancel messages in Deferred.