incusd/scriptlet: Make set_target fail with invalid members #2889

Job	Run time
Documentation	5m 15s
Code (oldstable)	7m 27s
Client (oldstable, ubuntu-latest)	38s
Code (stable)	7m 21s
Client (oldstable, macos-latest)	42s
System (stable, cluster, dir)	11m 49s
Code (tip)	10m 15s
System (stable, cluster, btrfs)	12m 10s
Client (oldstable, windows-latest)	1m 9s
System (stable, cluster, lvm)	12m 50s
Client (stable, ubuntu-latest)	46s
System (stable, cluster, zfs)	13m 56s
Client (stable, macos-latest)	46s
System (stable, cluster, ceph)	15m 52s
Client (stable, windows-latest)	1m 27s
System (stable, cluster, random)	12m 2s
System (stable, standalone, dir)	22m 25s
System (stable, standalone, btrfs)	23m 57s
System (stable, standalone, lvm)	39m 59s
System (stable, standalone, zfs)	39m 27s
System (stable, standalone, ceph)	1h 15m 25s
System (stable, standalone, random)	23m 18s
System (oldstable, cluster, dir)	13m 6s
System (oldstable, standalone, dir)	22m 17s
System (tip, cluster, dir)	14m 33s
System (tip, standalone, dir)	25m 3s
	6h 53m 55s

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