feat(xo-server): better info when failing to start a VM
feat(xo-server): better info when failing to start a VM

Fixes https://xcp-ng.org/forum/post/47329

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