wmllint: report stored units never unstored or cleared

Keys from the dictionary of stored units are removed as the unit is
unstored or its variable cleared. However, I found that one character in
Legend of Wesmere, Urudin, is stored but apparently never unstored/cleared.
I figure it's best to report such cases.

This message does not include helpful information like the filename,
because that data was never stored in the dictionary. The dictionary was
designed on the presumption that all entries would have a matching unstore/
clear event, and I didn't think of this warning until I saw there was an
anomalous case. Those who get this error will just have to use grep!