Fix onerror event triggering for fake xhr requests

[fearphage]

Purpose (TL;DR) - mandatory

The xhr.onerror event was not firing at all.

Background (Problem in detail) - optional

I cam across this issue while looking into #1040 and #1031. There are a finite number of xhr.on__event__ hooks that sinon was checking and onerror was not on the list.

Note: There is talk of errors being triggered at the wrong time in sinon. I did not address that at all. This only makes xhr.onerror trigger when an error event is triggered. I added additional tests to ensure the order and make sure this stays functional in the future.

Solution

Added error event to sequence of events looked for on xhr object (xhr['on' + name]).

How to verify - mandatory

1. Check out this branch (see github instructions below)
2. npm install
3. npm test