bootstrap: If dir_is_empty fails, show the non-existent directory path

This should help when trying to debug issues.

src/bootstrap/src/utils/helpers.rs

@@ -438,7 +438,7 @@ fn lld_flag_no_threads(builder: &Builder<'_>, lld_mode: LldMode, is_windows: boo
 }
 
 pub fn dir_is_empty(dir: &Path) -> bool {
-    t!(std::fs::read_dir(dir)).next().is_none()
+    t!(std::fs::read_dir(dir), dir).next().is_none()
 }
 
 /// Extract the beta revision from the full version string.