Add a "pause on error" command to the VM service

[nex3]

When debugging, it's very useful for a user to be able to pause the program when an error occurs. Of course there's already top-level error handling that can be done, but specifically being able to pause at the site where the error is thrown is most useful for inspecting local state and setting relevant breakpoints for a future run.

This is supported via Observatory, so I assume it's available privately in the VM service. Making it public would be great, and would help improve the debugging UI for the test runner.

[sethladd]

This would be great for Sky development, too.

[turnidge]

We have implemented the ability to break when any exception is thrown. This is currently implemented in the hidden method _setExceptionPauseInfo.

I agree that this service method should be made public.