Not all our errors actually happen in the contexts we were
wrapping (e.g. `setTimeout` and `_.throttle`). Also this fixes the
neat Firefox inspector feature that shows you where your event
handlers for a given DOM element actually live.
Using this "semi-modern" browser event means that Safari 9 and older
and IE10 and older may not have our browser error reporting active;
that seems fine giving the vanishing market share of those browsers.
https://blog.sentry.io/2016/01/04/client-javascript-reporting-window-onerror
Signed-off-by: Anders Kaseorg <anders@zulipchat.com>