capture fuller backtraces from js errors

Currently our onerror handler is only able to capture very location of js crash without any backtrace.

The later is very important to have. There are some way to get it. At very least we could start paying attention to error argument of onerror that's available on latest blink-based browsers. Fuller option is to generalize bugsnag code (which looks good but is currently tied too much with reporting crashes to bugsnag.com) and start using it.