Crash/non-informative behaviour when the update server is blocked by a filtering proxy

One of our clients had a filtering proxy that returned a message to the effect that the updates.drupal.org server was blocked as its use was unfit for a corporate environment (must be the titillating combination of the words "up", "date" and "release"). Instead of returning a normal error however it was returning a full HTML document -- like every single filtering proxy from a company rhyming with San Francisco I have ever seen --, which made the SimpleXMLParser (with Warnings on) crash with lots of error messages right away and without useful information. So this patch just adds a sanity check to make sure that the returned document is indeed XML and, if for any reason it is not, to display the same error message as if the serverw as unreachable (which is the case).

As the update process no longer crashes at the first check however, modules pulling update information from private URLs will have a chance (hopefully) to get, hm..., full satisfaction.