[ADD] mail_composer_cc_bcc: more helpful error message for test

This is a test that compares the hash of the source code of a function they
override with a list of known hashes, so as soon as this changes in upstream,
the test fails and we know we have to check if we need to incorporate any changes
from there.

mail_composer_cc_bcc/tests/test_mail_cc_bcc.py

@@ -48,7 +48,12 @@ def test_upstream_file_hash(self):
         """Test that copied upstream function hasn't received fixes"""
         func = inspect.getsource(upstream._send).encode()
         func_hash = hashlib.md5(func).hexdigest()
-        self.assertIn(func_hash, VALID_HASHES)
+        self.assertIn(
+            func_hash,
+            VALID_HASHES,
+            "mail.mail#_send has changed in upstream, "
+            "please adapt the override and add the new hash above",
+        )
 
     def test_email_cc_bcc(self):
         form = self.open_mail_composer_form()