Make it easier to separate errors from an actual answer, and ensure
we do a fallback check if there was an error.
Also fix the error code returned from aa_is_enabled() which got
broken by the addition of the private_enabled() check.
Finally make sure the private enabled error code is documented.
Fixes: https://gitlab.com/apparmor/apparmor/-/issues/150
MR: https://gitlab.com/apparmor/apparmor/-/merge_requests/713
Signed-off-by: John Johansen <john.johansen@canonical.com>