utils.log: Write error messages to stderr

When “gnt-cluster copyfile” failed it would only print “Copy of file …
to node … failed”. A detailed message is written using logging.error.
Writing error messages to stderr can be helpful in figuring out what
went wrong (the messages also go to the log file, but not everyone might
know about it).

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>