Fix: Cloudwatch delete Alarm status code handling on invalid alarm name (#3028)

* CloudWwatch delete Alarm status code handling on invalid alarm Name

* Handled cases where a mix of existent and non existent alarms are tried to delete

* Linting

Co-authored-by: usmankb <usman@krazybee.com>
Co-authored-by: Bert Blommers <info@bertblommers.nl>

moto/cloudwatch/models.py

@@ -304,6 +304,13 @@ class CloudWatchBackend(BaseBackend):
         )
 
     def delete_alarms(self, alarm_names):
+        for alarm_name in alarm_names:
+            if alarm_name not in self.alarms:
+                raise RESTError(
+                    "ResourceNotFound",
+                    "Alarm {0} not found".format(alarm_name),
+                    status=404,
+                )
         for alarm_name in alarm_names:
             self.alarms.pop(alarm_name, None)