lxd/device/errors: Adds ErrCannotUpdate error
lxd/device/errors: Adds ErrCannotUpdate error

Return by the Update() function if the update is not supported by the device based on the instance type or run state.

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

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