frontend mail rule validation

Display non-field validation errors, hide action param field if not needed

src/documents/serialisers.py

@@ -785,3 +785,12 @@ class MailRuleSerializer(serializers.ModelSerializer):
         if assign_tags:
             mail_rule.assign_tags.set(assign_tags)
         return mail_rule
+
+    def validate(self, attrs):
+        if (
+            attrs["action"] == MailRule.MailAction.TAG
+            or attrs["action"] == MailRule.MailAction.MOVE
+        ) and attrs["action_parameter"] is None:
+            raise serializers.ValidationError("An action parameter is required.")
+
+        return attrs