incusd/migration: Properly forward errors #375

Job	Run time
Code	7m 18s
Documentation	7m 0s
System (1.20.x, cluster, dir)	11m 3s
Client (1.20.x, ubuntu-latest)	30s
System (1.20.x, cluster, btrfs)	11m 50s
Client (1.20.x, macos-latest)	49s
System (1.20.x, cluster, lvm)	1m 42s
Client (1.20.x, windows-latest)	1m 38s
System (1.20.x, cluster, zfs)	13m 30s
System (1.20.x, cluster, ceph)	15m 31s
System (1.20.x, cluster, random)	10m 55s
System (1.20.x, standalone, dir)	25m 30s
System (1.20.x, standalone, btrfs)	29m 8s
System (1.20.x, standalone, lvm)	39m 48s
System (1.20.x, standalone, zfs)	42m 53s
System (1.20.x, standalone, ceph)	1h 14m 9s
System (1.20.x, standalone, random)	22m 57s
System (stable, cluster, dir)	11m 21s
System (stable, standalone, dir)	21m 4s
System (tip, cluster, dir)	11m 40s
System (tip, standalone, dir)	21m 46s
	6h 22m 2s

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