AutoMod: error if invalid search check for type

If all of the fields in a search check are invalid for a type, we should
throw an error instead of just quietly disregarding that search check.
The previous behavior caused a rule with, for example, separate searches
for both "domain" and "body" to apply to comments, even though the
presence of a domain check should have caused it to be restricted to
only submissions.

r2/r2/lib/automoderator.py

@@ -620,6 +620,12 @@ class RuleTarget(object):
         valid_fields = self._match_fields_by_type[self.target_type]
         fields = {field for field in fields if field in valid_fields}
 
+        if not fields:
+            raise AutoModeratorRuleTypeError(
+                "Can't search `%s` on this type" % key,
+                self.parent.yaml,
+            )
+
         modifiers = matches.group(2)
         if modifiers:
             modifiers = [mod.strip() for mod in modifiers.split(",")]