Avoid repeated lag-generating continuous attempts to retrieve HG service Urls in UMM if the initial request fails, possibly due to the home URL not being present

Instead, the failure information is effectively added to the memory cache (so no persistence over simualtor sessions).
A future improvement may be to invalidate negative cache results after some time has passed in case the failure was transient.
Looks to resolve http://opensimulator.org/mantis/view.php?id=7382