WIP: Applied feedback

See also:
- Part 1: #12703
- Part 2: #12808
- Adding an error code for suspended accounts #12845

Author: Yoric

changelog.d/12846.feature

@@ -0,0 +1 @@
+Experimental: expand `check_event_for_spam` with ability to return additional fields. This enables spam-checker implementations to experiment with mechanisms to give users more information about why they are blocked and whether any action is needed from them to be unblocked.

changelog.d/12846.misc

@@ -888,14 +888,21 @@ async def create_and_send_nonmember_event(
 
             spam_check = await self.spam_checker.check_event_for_spam(event)
             if spam_check is not synapse.spam_checker_api.Allow.ALLOW:
-                if isinstance(spam_check, tuple):
-                    [code, dict] = spam_check
-                    raise SynapseError(
-                        403,
-                        "This message had been rejected as probable spam",
-                        code,
-                        dict,
+                try:
+                    if isinstance(spam_check, tuple):
+                        [code, dict] = spam_check
+                        raise SynapseError(
+                            403,
+                            "This message had been rejected as probable spam",
+                            code,
+                            dict,
+                        )
+                except ValueError:
+                    logger.warning(
+                        "Spam-check module returned invalid error value. Expecting [code, dict], got %s",
+                        spam_check,
                     )
+                    spam_check = Codes.FORBIDDEN
                 raise SynapseError(
                     403, "This message had been rejected as probable spam", spam_check
                 )