Allow the exception reason to pass through to the onErrorItem and onCompleteItem callbacks.

I think it is useful since it allows the client to figure out what could have failed.

I was attempting to add invalid header data. In this case adding an emoji to a header value ("😀") results in e being:

code: 12,
message:"Failed to execute 'setRequestHeader' on 'XMLHttpRequest': '😀' is not a valid HTTP header field value."
name:"SyntaxError"
stack: <removed>

If I could detect code 12 then I'd be able to address it to the user correctly. A very chance of occurring but a chance nonetheless.