don't crash when failed to determine context

This changes output when using brief reported from `TypeError: uncaughtException listener threw error: Cannot read property 'description' of undefined` to `Uncaught exception in Unknown context: undefined`

Achievement unlocked.

Seriously though, this prevents a brief reporter crash, but still fails to report an actual error. Neither of the other reports do, because in the particular case when the browser throws _before_ any tests, the UncaughtError comes back inside the `e.data` - and I am not sure what to do in this case: should I check if `e.data` exists and pass that into `formatStack()`?

This is part of https://github.com/busterjs/buster/issues/422