utils crash if logfile does not exist

phablet@ubuntu-phablet:~/qrt-test-apparmor$ ls -l /var/log/syslog
ls: cannot access /var/log/syslog: No such file or directory

phablet@ubuntu-phablet:~/qrt-test-apparmor$ aa-enforce --help
Traceback (most recent call last):
  File "/usr/sbin/aa-enforce", line 17, in <module>
    import apparmor.tools
  File "/usr/lib/python3/dist-packages/apparmor/tools.py", line 17, in <module>
    import apparmor.aa as apparmor
  File "/usr/lib/python3/dist-packages/apparmor/aa.py", line 4641, in <module>
    raise AppArmorException('Can\'t find system log "%s".' % (filename))
apparmor.common.AppArmorException: 'Can\'t find system log "/var/log/syslog".'

While in general, systems should be in a state where the required logfile exists, the tools should not crash if they don't, due to a system problem or misconfiguration in /etc/apparmor/logprof.conf.