backup: avoid bubbling up first ECANCELED error

backup: avoid bubbling up first ECANCELED error

With pvebackup_propagate_error(), the first error wins. When one job
in the transaction fails, it is expected that later jobs get the
ECANCELED error. Those are not interesting and by skipping them a more
interesting error, which is likely the actual root cause, can win.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>