feat: update events remove_targets API response (#3340)

* feat: update events remove_targets API response

* test: add missing test for expected exception

moto/events/models.py

@@ -368,9 +368,12 @@ class EventsBackend(BaseBackend):
 
         if rule:
             rule.remove_targets(ids)
-            return True
-
-        return False
+            return {"FailedEntries": [], "FailedEntryCount": 0}
+        else:
+            raise JsonRESTError(
+                "ResourceNotFoundException",
+                "An entity that you specified does not exist",
+            )
 
     def test_event_pattern(self):
         raise NotImplementedError()

moto/events/responses.py

@@ -238,7 +238,10 @@ class EventsHandler(BaseResponse):
                 "ResourceNotFoundException", "Rule " + rule_name + " does not exist."
             )
 
-        return "", self.response_headers
+        return (
+            json.dumps({"FailedEntryCount": 0, "FailedEntries": []}),
+            self.response_headers,
+        )
 
     def test_event_pattern(self):
         pass