to have problems sending a control message (returns an "I/O error").
At least try to recover gracefully in the bening case where the user
did not request a particular serial number, so we could continue
anyway without knowing it.
git-svn-id: svn://svn.savannah.nongnu.org/avrdude/trunk@481 81a1dc3b-b13d-400b-aceb-764788c761c2