h4h5tools

Suppress bogus error messages when an error occurs.

Bogus error messages have been being printed. Although they are printed only when there is an error, those bogus messages can mislead users and even developers.

h4toh5 sometimes tries a special conversion, and tries another way if the first trial fails. During the first stage, error messages are accumulated even though they don't cause errors. This changelist clears those bogus messages.