session-lockdown: provide debug information about processes we look at

We're effectively asserting that all these processes are running,
so we should look at whether they, in fact, *are* running, and
if not, why not.

Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D3639