New error and progress callbacks for the olMap component

The implementation in components/olmap/olmap.js should be complete, but the UI is not properly updated in the callbacks in maps/detail/compose.js.
Not as long as we continue to load the image by setting the src of an img element. Instead, we could use XHR to load the image, but that would fail on older IE versions. Then we could return the XML that contains the error message if the returned content type was not image/whatever.

Note that using XHR to load images adds a bit of overhead. So it might even be better to re-request the image using XHR if there was an error.