Report unhandled promise rejections to devtools

[jdm]

When there's no explicit unhandledrejection listener, we should do useful things like printing a message to stdout and sending a message to the devtools. We should also see if Firefox does anything smart here.

[jdm]

For what it's worth, a user script containing:

onunhandledrejection = function(e) {
    throw e;
};

yields very helpful output on pages that are otherwise uninformative sometimes.