I had to look at this because my application was hanging indefinitely using this particular XHR lib -- I had a domain name spelled wrong, and neither the error() nor load() events were firing, so my Promise was never resolving nor rejecting.

This fix addresses that problem by correcting the firing order (and ability to fire) of load, loadend, abort and error events. It also fixes the mis-fire of abort in the current driverdan 1.8 (original repo) version of this code that re-presented itself when I fixed abort firing.

A few smaller fixes were made while debugging this; in particular, status is supposed to be zero when the error handler fires, but was set to 503. I left it at 503 for the extra-process request stuff since I didn't have time to give that enough thought.

Also, I made exceptions into instanceof Error so that stack traces work (already done in driverdan code), and I threw all the event firing once readyState=4 on the NodeJS event loop. I left the earlier events alone since putting everything on the event loop was causing firing order issues; no readyState transistions were firing until the readyState was 4. I expect this change will let multiple load/error/abort handlers which await other (non-xhr) events to interoperate better with no correctness impact.