coredump: Don't log an error if D-Bus isn't running

coredumpctl could be used in a chroot where D-Bus isn't running. If
that's the case, we shouldn't consider it an error if we can't connect
to the D-Bus daemon so let's reduce the severity of the error we log
when we can't connect to D-Bus because the socket doesn't exist.

(cherry picked from commit 414bd2e7)
(cherry picked from commit 6745eaa6)