session-lockdown-no-deny: only ignore FileNotFoundError

If we fail to read the AppArmor profile or other required information
due to a time-of-check/time-of-use difference (the process exits)
then that's fine. Otherwise, it's a problem and we should fail,
although we might as well continue testing and get more complete
results.

Reviewed-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D3770