irkerd: Replace sys.stderr.write with LOG.error

irkerd: Replace sys.stderr.write with LOG.error

Thie makes logging more consistent with other errors (e.g. you can
adjust the logging Handler and get all the errors sent to syslog or a
file).  I also removed the 'irkerd: ' prefix; if we want that, I think
we should add it to all logged messages by using a custom string in
the logging Formatter.