sd-bus: make sure propagate all errors with vtable callbacks back to 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: #6059