Change #8162: Conversion of Town Road, Impr. Error

This changes the error when you attempt to convert a road owned by a town to another road, specifying that it's owned by the town rather than simply being incompatible. As the original poster of the issue pointed out, these seemed to be the only occurences of the incompatible road string, so now it's unused, but they would be left untouched in case of future use or since changing it to a different error would do the work of removing it then. If requested, it likely wouldn't be too difficult to remove the string entirely.