Alerting: recognize Cortex datasources correctly in the frontend (#44316)

* Alerting: always use msg field for user facing errors

* fix: revert front-end Cortex detection

Co-authored-by: gillesdemey <gilles.de.mey@gmail.com>

pkg/services/ngalert/api/util.go

@@ -265,7 +265,7 @@ func ErrResp(status int, err error, msg string, args ...interface{}) *response.N
 	if msg != "" {
 		err = errors.WithMessagef(err, msg, args...)
 	}
-	return response.Error(status, "API error", err)
+	return response.Error(status, err.Error(), err)
 }
 
 // accessForbiddenResp creates a response of forbidden access.