The underlying issue is that a zpool can get imported only once, so
we first check if it's in `zpool list`, and thus imported, and only
if it does not shows up there we try to import it.
But, this can race with either:
* parallel running activate_storage call, through CLI/API/daemon
* a zpool import from an admin (a bit unlikely, but hey that's the
thing with race conditions ;))
So refactor the "is pool imported" check into a closure, and call it
addditionally if the import failed, and silent the error if the pool
is now listed, and thus imported. This makes it a little bit nicer to
read too, IMO.
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>