ceph, config, auth: better messages on failure to open keyring/ceph.conf

If something as simple as file ownership is wrong, Ceph commands
and daemons can fail to run, and the diagnostics are not great.
Improve that for at least the specific cases of unopenable keyring and
ceph.conf files.

Fixes: #5634
Signed-off-by: Dan Mick <dan.mick@inktank.com>