Add errorsource to InfluxDB datasource plugin fixes #1072 (#99900)

This PR adds `backend.ErrorSourceDownstream` values to all `backend.DataResponse`
values where it's certain that the error wasn't the result of the
InfluxDB datasource plugin.

pkg/tsdb/influxdb/fsql/query_model.go

@@ -48,7 +48,8 @@ func getQueryModel(dataQuery backend.DataQuery) (*queryModel, error) {
 		Format:        format,
 	}
 
-	// Process macros and execute the query.
+	// Process macros and generate raw fsql to be sent to
+	// influxdb backend for execution.
 	sql, err := sqlutil.Interpolate(query, macros)
 	if err != nil {
 		return nil, fmt.Errorf("macro interpolation: %w", err)