allow clicking on labels in error messages to be taken to that point-in-time

[nikomatsakis]

It'd be really cool if, when you get an error that says something like "permission revoked here", you could click on the error to be taken to the state of the program at that point.

RIght now, our breakpoints assume no loops etc, to really make this nice we'd have to augment that with a better notion of backtracking, but I guess it would work "ok" for now to just take you to the PC of the span.