loadFeaturesXhr fcn.: add failure callback and return xhrIo request.
loadFeaturesXhr fcn.: add failure callback and return xhrIo request.

- So that callers can keep track of pending XHR requests and cleanup
  accordingly, in addition to triggering a success() callback when
  the XHR request completes, loadFeaturesXhr now triggers a failure()
  callback when the XHR request completes with an error.
- The fcn. also returns the XHR request so callers can manage them
  better, e.g., by calling abort() if they so desire.
- Updated the declaration of postBody to allow null|undefined, since
  not all callers use postBody.

landonb/ol3,landonb/ol3,landonb/ol3,landonb/ol3