Fix crash when no log in supported location can be found

This can happen for example on systems that use Journal
from systemd.

In this case, ufw-gtk exits with a traceback containing:
IOError: [Errno 2] No such file or directory: '/var/log/messages.log'
(this is the last log file tried).

The patch works around the issue by handling the error
and disabling the widget in the "Events" tab.


(Journal support would be welcome as well.)
This commit fixes issue 21 on ufw-frontends tracker.