Support --pause-on-error in Chrome and PhantomJS

[nex3]

This is a very useful flag for debugging. With #294 and #296 it isn't strictly necessary, since the user can just set it manually via the dev tools, but it's much easier to set it via the command-line.

This will require starting and connecting to the Chrome dev tools and sending Debugger.setPauseOnExceptions before we begin loading or running tests. I'm reasonably sure that PhantomJS uses a similar protocol, since it's WebKit based and its remote debugger is Chrome-compatible, but we'll have to make sure.

It's also possible that this could work with IE using the IE Diagnostics Adapter.

[grouma]

Ideally we should make use of google/webkit_inspection_protocol.dart

However, allowing the user to interact with the chrome dev tools through the UI and WIP at the same time is blocked by https://bugs.chromium.org/p/chromium/issues/detail?id=129539.

[grouma]

Looks like the blocking bug was resolved.