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

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`](https://chromedevtools.github.io/devtools-protocol/tot/Debugger/#method-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](https://github.com/Microsoft/IEDiagnosticsAdapter/).

No files or tests are named. Start by tracing command-line flag handling and the Chrome and PhantomJS remote-debugger connection, then verify how Debugger.setPauseOnExceptions can be sent before tests load or run. Done means --pause-on-error works in Chrome and PhantomJS, with PhantomJS protocol compatibility confirmed.

Written by the indexing model from the issue text.