Add stack trace onerror when error object is available

[apackin]

#24

Most browsers now support the Error object as a 5th parameter to onerror. PR uses the error object to include a stack trace as per the issue above.

[fastfrwrd]

This would be pretty useful for us, and I like the simplicity of it. A more cross-browser solution would turn out to be pretty involved: https://blog.sentry.io/2016/01/04/client-javascript-reporting-window-onerror.html

[mostlyjason]

Thanks for the suggestion! We will review this next time we do an update of the library.