fact that key doesn't exist when going to delete it. Otherwise, error could lead to installer aborting when it should just treat the delete as a noop. a=valeski, r=hewitt, sr=dveditz, caffeine=pepsi. git-svn-id: svn://10.0.0.236/branches/MOZILLA_1_0_BRANCH@127663 18797224-902f-48f8-a5cc-f745e15eee43