Destroy "rescue nil"!

* Updated to a new version of Rubel which had the "ErrorHandler" removed. All this did was catch and re-raise an exception, in the process wiping out the backtrace.

* Removed most (all?) instances of `rescue nil` from the Qernel and GQL namespaces. Exceptions will now be allowed to "bubble up" to the ScenarioPresenter, and a full (useful!) backtrace will be available.

* When possible, the ActiveDocument in which a query is stored [will be included in the backtrace](https://f.cloud.github.com/assets/4383/2167551/edc665a0-951b-11e3-9b2a-7027ad0623ae.png).

* `fetch_and_rescue` no longer rescues, and has been renamed `fetch`.

Assigning to @dennisschoenmakers for review.