Don't support "Data" as a source location for fomods

Fomods that pull files from <fomod_folder>/Data aren't supported since
we have to delete the Data folder when we reconfigure them. Produce a
useful error when this happens to indicate manual intervention is
required.

So far, this impacts new versions of MCM Helper. There might or might
not be others.

ammo/fomod_controller.py

@@ -367,6 +367,13 @@ def _install_files(self, selected_nodes: list) -> None:
             full_source = self.mod.location
             for i in s.split("\\"):
                 folder = i
+                assert (
+                    # If a fomod has a Data folder in its normal structure and uses
+                    # it as file sources, it will break the second time you configure it.
+                    # This isn't supported. Just advise people to reinstall the mod.
+                    full_source.exists()
+                ), "This fomod uses Data as a source folder (unsupported). Reinstall then configure manually."
+
                 for file in os.listdir(full_source):
                     if file.lower() == i.lower():
                         folder = file