feat(xo-server/vm.migrate): ensure original error is logged
feat(xo-server/vm.migrate): ensure original error is logged

Fixes https://github.com/vatesfr/xen-orchestra/pull/4364/files#r488539823

lmcro/xo-web,vatesfr/xo-web,lmcro/xo-web,lmcro/xo-web,vatesfr/xo-web