…clients

Previously we'd propagate errors returned by user callbacks configured
in vtables back to the users only for method handlers and property
get/set handlers. This does the same for child enumeration and when we
check whether a fallback unit exists.

Without this the failure will be treated as a non-recoverable connection
error and result in connection termination.

Fixes: systemd#6059

…istent unit

See: systemd#6059