<exception> variable should be available at all levels of the stack

Currently when an exception occurs, we inject an `` variable into the top stack frame. This means if you navigate up the stack (eg. if the exception occurred inside the `assert` implementation but you want to view the code that contained the condition) it's not longer visible.

It'd be better if this variable was "global" while you were paused on the exception so you can see it when at any level of the stack.