bugtool: show error on Copy in tarAddFile

We could also use CopyN using fileSrcInfo.Size() but it would silently
copy empty files in the case of cgroup fs for example. Let's just
display an error so that we can see what failed.

Signed-off-by: Mahe Tardy <mahe.tardy@gmail.com>

pkg/bugtool/bugtool.go

@@ -185,7 +185,7 @@ func (s *bugtoolInfo) tarAddFile(tarWriter *tar.Writer, fnameSrc string, fnameDs
 
 	_, err = io.Copy(tarWriter, fileSrc)
 	if err != nil {
-		s.multiLog.WithField("fnameSrc", fnameSrc).Warn("error copying data from source file")
+		s.multiLog.WithError(err).WithField("fnameSrc", fnameSrc).Warn("error copying data from source file")
 		return err
 	}