stacktraceErr should take presidence over err

[blittle]

https://github.com/CanopyTax/auto-trace/blob/master/src/auto-trace.js#L101

When I do the following:

const trace = throwAsyncStacktrace();
setTimeout(() => {
  trace('some stringed error');
}, 1000);

The stack trace for the original trace error is ignored in favor of the error created inside the timeout.

[joeldenning]

Fixed with #3