refactor(rm): improve error context in check_one_fs

Replace empty error strings with descriptive messages when path parent
or mount point resolution fails. This aids in debugging by identifying
specifically which path caused the resolution failure during the
one-file-system check.

Author: shskwmt

src/uu/rm/src/rm.rs

@@ -750,11 +750,16 @@ fn check_one_fs(path: &Path, options: &Options) -> Result<(), String> {
         .map_err(|err| format!("cannot canonicalize {}: {err}", path.quote()))?;
 
     // Get parent path, handling root case
-    let parent_canon = child_canon.parent().ok_or("")?.to_path_buf();
+    let parent_canon = child_canon
+        .parent()
+        .ok_or_else(|| format!("cannot get parent of {}", child_canon.quote()))?
+        .to_path_buf();
 
     // Find mount points for child and parent
-    let child_mount = mount_for_path(&child_canon, &fs_list).ok_or("")?;
-    let parent_mount = mount_for_path(&parent_canon, &fs_list).ok_or("")?;
+    let child_mount = mount_for_path(&child_canon, &fs_list)
+        .ok_or_else(|| format!("cannot find mount point for {}", child_canon.quote()))?;
+    let parent_mount = mount_for_path(&parent_canon, &fs_list)
+        .ok_or_else(|| format!("cannot find mount point for {}", parent_canon.quote()))?;
 
     // Check if child and parent are on the same device
     if child_mount.dev_id != parent_mount.dev_id {