lxc storage volume edit should fail when editing snapshots

[MggMuggins]

I suspect that snapshot volumes cannot have their configuration changed (although I haven't confirmed it). The following command should probably fail and print an error.

Steps to reproduce

storage volume edit opens the editor, and returns 0 regardless of the changes made in the yaml:

$ lxc storage volume edit default virtual-machine/vm1/snap0
$ echo $?
0

None of the changes made during the edit are persisted (lxc storage volume show virtual-machine/vm1/snap0).