Alerting: Return 404 when /api/ruler/grafana/api/v1/rules/{Namespace}/{Groupname} does not exist (#100264)

* Return a 404 when rule group doesn't exist

* Update tests

* Update swagger doc and tests

pkg/services/ngalert/api/tooling/definitions/cortex-ruler.go

@@ -167,6 +167,7 @@ import (
 //     Responses:
 //       202: RuleGroupConfigResponse
 //       403: ForbiddenError
+//		 404: NotFound
 
 // swagger:route Get /ruler/{DatasourceUID}/api/v1/rules/{Namespace}/{Groupname} ruler RouteGetRulegGroupConfig
 //