lxd/cluster/upgrade: Don't panic in UpgradeMembersWithoutRole in raft ID not found
lxd/cluster/upgrade: Don't panic in UpgradeMembersWithoutRole in raft ID not found

Panicking in servers is bad because it takes down the whole service, meaning the admin cannot fix the issue.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

stgraber/lxd,lxc/lxd,stgraber/lxd,hallyn/lxd,hallyn/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,hallyn/lxd,hallyn/lxd