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.

src/coredump/coredumpctl.c

@@ -1180,6 +1180,10 @@ static int check_units_active(void) {
                 return false;
 
         r = sd_bus_default_system(&bus);
+        if (r == -ENOENT) {
+                log_debug("D-Bus is not running, skipping active unit check");
+                return 0;
+        }
         if (r < 0)
                 return log_error_errno(r, "Failed to acquire bus: %m");