Print a log message if the sender command fails.

Print a log message if the sender command fails.

Currently there is no infrastructure for passing down a value from vss,
which runs in parent context, to the child process. In particular,
we can not pass the return value of the sender subcommand handlers
to com_sender() to propagate it to the client.

However, we may easily write a log message that explains the cause
of the error, which is not optimal, but better than nothing.

Thanks to Gerrit Renker who pointed out this flaw.