Warn if the game directory is empty or invalid

Currently if you don't have it configured (or it's invalid for
wahtever other reason) you get an unhelpful "failed to read Item
EXH" error. Now there is a clearer warning message, which also
prints out the current configuration value.

This is a warning, not an error - in case someone is intentionally
trying to run Kawari without a game path specified.

src/common/gamedata.rs

@@ -74,6 +74,14 @@ impl GameData {
             SqPackResource::from_existing(Platform::Win32, &config.filesystem.game_path),
             &config.filesystem.unpack_path,
         );
+
+        if sqpack_resource.sqpack_resource.repositories.is_empty() {
+            tracing::warn!(
+                "You have an empty game directory ({:?}). This may be a configuration issue, you may want to read the usage documentation.",
+                config.filesystem.game_path
+            );
+        }
+
         let mut resource_resolver = ResourceResolver::new();
         for path in config.filesystem.additional_search_paths {
             let unpacked_resource = UnpackedResource::from_existing(&path);