feat($http): allow differentiation between XHR completion, error, abort, timeout

Previously, it wasn't possible to tell if an `$http`-initiated XMLHttpRequest
was completed normally or with an error or it was aborted or timed out.
This commit adds a new property on the `response` object (`xhrStatus`) which
allows to defferentiate between the possible statuses.

Fixes #15924

Closes #15847