emoji_dump: Add warning for fallback to black/white.

I don't think the black/white fallback code is actually used by
default, and thus when it is used, it's usually a sign that something
is broken.  We should throw an error, but at the very least it makes
sense to print a warning.

tools/setup/emoji_dump/emoji_dump.py

@@ -104,6 +104,7 @@ def main():
         try:
             color_font(code_point, code_point_to_fname_map)
         except MissingGlyphError:
+            print("Warning: Missing color glyph for %s; using black/white." % (name,))
             try:
                 bw_font(name, code_point)
             except Exception as e: