Add custom matcher for rejecting a promise

[sjelin]

jasminewd is great when promises resolve, but but it can be tough to test rejected promises for a few reasons:

• expect will try to unwrap the promise, but doesn't have code to handle rejections. This will also get in the way of any user trying to write a custom matcher
• If a WebDriver command fails and there's no rejection handler set up yet, WebDriver will crash the flow.execute command we're wrapping each it() block in.
• Userers are used to promises being handled for them ✨magically✨

So we should add a custom marcher toBeRejected