Provide more information on a timeout error

We see build errors pretty often like:

https://travis-ci.org/canjs/canjs/builds/253644821#L3694

```
Error: Browser timed out within 120 seconds
at Timeout.testTimedOut [as _onTimeout] (/home/travis/build/[secure]/[secure]/node_modules/testee/lib/runner.js:47:27)
at ontimeout (timers.js:386:14)
at tryOnTimeout (timers.js:250:5)
at Timer.listOnTimeout (timers.js:214:5)
```

I'm not sure if this is testee, or our build. One way to tell is if testee encounters a timeout error, it should scrape the `HTML` content of the page and print it (or even a subset of it) for the user. And also may forward any caught `window.onerror` messages.

Start with the linked Travis build and the timeout stack trace at node_modules/testee/lib/runner.js:47 to determine how testee handles browser timeouts. Done means a timeout reports useful page HTML or a relevant subset and forwards any caught window.onerror messages.

Written by the indexing model from the issue text.