Fix breaking event sending due to bad push rule

Broke by #13522

It looks like we have some rules in the DB with a priority class less
than 0 that don't override the base rules. Before these were just
dropped, but #13522 made that a hard error.

synapse/push/baserules.py

@@ -49,13 +49,16 @@
 """
 
 import itertools
+import logging
 from typing import Dict, Iterator, List, Mapping, Sequence, Tuple, Union
 
 import attr
 
 from synapse.config.experimental import ExperimentalConfig
 from synapse.push.rulekinds import PRIORITY_CLASS_MAP
 
+logger = logging.getLogger(__name__)
+
 
 @attr.s(auto_attribs=True, slots=True, frozen=True)
 class PushRule:
@@ -199,6 +202,12 @@ def compile_push_rules(rawrules: List[PushRule]) -> PushRules:
             collection = rules.sender
         elif rule.priority_class == 1:
             collection = rules.underride
+        elif rule.priority_class <= 0:
+            logger.info(
+                "Got rule with priority class less than zero, but doesn't override a base rule: %s",
+                rule,
+            )
+            continue
         else:
             raise Exception(f"Unknown priority class: {rule.priority_class}")