CloudWatch/Logs: Fix log alerts in new unified alerting (#36558)

* Pass FromAlert header from new alerting

* Add better error messages

pkg/expr/transform.go

@@ -206,6 +206,7 @@ func (s *Service) queryData(ctx context.Context, req *backend.QueryDataRequest)
 	tQ := plugins.DataQuery{
 		TimeRange: &timeRange,
 		Queries:   queries,
+		Headers:   req.Headers,
 	}
 
 	// Execute the converted queries