When attempting to configure either the listen-interface or the upstream-interface and the interface doesn't exist, the isc-dhcp-relay.service fails to start.

This is likely expected behavior, but there should be some checks in verify() for this to be handled gracefully.

Error:

vyos@vyos# commit
[ service dhcp-relay ]
Job for isc-dhcp-relay.service failed because the control process exited with error code.
See "systemctl status isc-dhcp-relay.service" and "journalctl -xeu isc-dhcp-relay.service" for details.