Don't report that properties have changed if processing of an evented updated files to complete.  Since a failure may have left a proxy in an inconsistent state (variables that normally update together may be out of sync), resubscribe to force an update of all properties.
Throw PropertyError rather than asserting if Value() is called on a property that has never been updated.  This makes it possible to use the (any)PropertyChange callback with UPnP forum services which may have optional state variables.
Merge branch 'master' of ssh://core.linn.co.uk/home/git/ohnet