better source listing & minor fixes

This is a continuation of https://github.com/campd/fxdt-adapters/pull/81 (coming from master now, had to open a new PR)

This makes the source listing a lot more reliable. It will clear when navigating across pages, and you'll get the right source list back after a `reconfigure` which clears the list. This also makes it you'll always see the sources when the debugger first loads.

There's a few other small fixes in here too for dealing with bad URLs and frames with an undefined `this`. Been testing mostly on Chrome but will thoroughly test on iOS now.