address comments

Author: elainezhao1

toolkit/tools/pkg/imagecustomizerlib/cosicommon.go

@@ -178,7 +178,10 @@ func buildCosiFile(sourceDir string, outputFile string, partitions []outputParti
 }
 
 func addToCosi(data ImageBuildData, tw *tar.Writer) error {
-	addFileToCosi(tw, data.Source, data.Metadata.Image)
+	err := addFileToCosi(tw, data.Source, data.Metadata.Image)
+	if err != nil {
+		return fmt.Errorf("failed to add image file to COSI:\n%w", err)
+	}
 
 	if data.VeritySource != "" && data.Metadata.Verity != nil {
 		err := addFileToCosi(tw, data.VeritySource, data.Metadata.Verity.Image)