Provide more information when a filewatcher failure is hit. (#13715)

A naked unwrap led to an opaque error that can be hit when using the
embedded filewatcher.

I've changed this an unwrap_or_else panic! with the error message
providing more details about the failed operation.

A better solution would be to print an error! and not panic...

This was tested with the asset_processing example.

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>

crates/bevy_asset/src/io/file/file_watcher.rs

@@ -46,7 +46,13 @@ impl FileWatcher {
 impl AssetWatcher for FileWatcher {}
 
 pub(crate) fn get_asset_path(root: &Path, absolute_path: &Path) -> (PathBuf, bool) {
-    let relative_path = absolute_path.strip_prefix(root).unwrap();
+    let relative_path = absolute_path.strip_prefix(root).unwrap_or_else(|_| {
+        panic!(
+            "FileWatcher::get_asset_path() failed to strip prefix from absolute path: absolute_path={:?}, root={:?}",
+            absolute_path,
+            root
+        )
+    });
     let is_meta = relative_path
         .extension()
         .map(|e| e == "meta")