rm: use "traversal failed" message when readdir fails during recursive removal

When readdir fails mid-traversal (e.g., due to I/O errors), GNU rm
reports "traversal failed: <dir>" rather than "cannot remove '<dir>'".
Update the error handling in safe_remove_dir_recursive_impl to match
this behavior.

Fixes GNU test: tests/rm/rm-readdir-fail.sh

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: kevinburkesegment

tests/by-util/test_rm.rs

@@ -8,6 +8,8 @@
 use std::process::Stdio;
 
 use uutests::{at_and_ucmd, new_ucmd, util::TestScenario, util_name};
+#[cfg(unix)]
+use rlimit::Resource;
 
 #[test]
 fn test_invalid_arg() {
@@ -1377,3 +1379,23 @@ fn test_preserve_root_literal_root() {
         .stderr_contains("it is dangerous to operate recursively on '/'")
         .stderr_contains("use --no-preserve-root to override this failsafe");
 }
+
+/// Test that "traversal failed" message is shown when readdir fails during
+/// recursive removal (e.g., due to file descriptor exhaustion).
+#[cfg(unix)]
+#[test]
+fn test_traversal_failed_on_readdir_error() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    at.mkdir_all("a/b");
+    at.touch("a/b/file");
+
+    // Use a very low file descriptor limit so that dup() inside readdir
+    // fails with EMFILE ("Too many open files"), triggering the
+    // "traversal failed" error path.
+    let result = ucmd
+        .args(&["-rf", "a"])
+        .limit(Resource::NOFILE, 5, 5)
+        .fails();
+    result.stderr_contains("traversal failed");
+    result.stderr_contains("Too many open files");
+}