remove_files.py: Print warnings on removals

This will make it easier to debug build issues happening because if this
script in the future.

Bug: chromium:1314527
Change-Id: I5bbb4082ac816f98860e621917bc35b26ba7cb29
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/87700
Auto-Submit: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>

generator/remove_files.py

@@ -78,6 +78,7 @@ def run():
 
         for candidate in get_all_files_in_dir(stale_dir):
             if not check_is_allowed(candidate, allowed_dirs):
+                print("Warning: remove_files.py removed " + candidate)
                 os.remove(candidate)
 
     # Finished! Write the stamp file so ninja knows to not run this again.