when debug flag set, print stack trace to console

I've got myself into a bit of a mess with the original restore for I was using to add features/fix things. So, to make things simpler, I've made a new fork and will manually import my changes one at a time of separate branches and submit them to you.
As it's reasonable to assume whoever uses the debug flag also uses a modern browser, you can actually throw an error here using `error(message)`. See https://developer.mozilla.org/en-US/docs/Tools/Web_Console?redirectlocale=en-US&redirectslug=Using_the_Web_Console#Error_messages