Make it an error to unregister a locator strategy that hasn't been registered #961

Currently if you try to unregister a locator strategy that doesn't exist, there's only a notification in the log file that no such strategy exists and nothing is done. This can lead to problems if, for example, there's a typo in the strategy name and the actual strategy is accidentally not unregistered. Better make this situation an error so that users see if there has been a problem.