If input zip file cannot opened, print its name.

Test: touch empty.zip; merge_zips out.zip empty.zip
(output should contain `empty.zip: zip: not a valid zip file`)

Change-Id: If707876af2301d7ad7d065c004cffb0c2491c7df

cmd/merge_zips/merge_zips.go

@@ -667,8 +667,10 @@ func (fiz *FileInputZip) Open() error {
 		return nil
 	}
 	var err error
-	fiz.reader, err = zip.OpenReader(fiz.Name())
-	return err
+	if fiz.reader, err = zip.OpenReader(fiz.Name()); err != nil {
+		return fmt.Errorf("%s: %s", fiz.Name(), err.Error())
+	}
+	return nil
 }
 
 func main() {