[check_port_free] Fix a race condition
[check_port_free] Fix a race condition

When the process is already dead (because it crashed / we killed its parent / we killed it for another port), do not raise the error

liqd/adhocracy,liqd/adhocracy,DanielNeugebauer/adhocracy,liqd/adhocracy,alkadis/vcv,phihag/adhocracy,alkadis/vcv,DanielNeugebauer/adhocracy,liqd/adhocracy,phihag/adhocracy,phihag/adhocracy,DanielNeugebauer/adhocracy,SysTheron/adhocracy,DanielNeugebauer/adhocracy,DanielNeugebauer/adhocracy,SysTheron/adhocracy,alkadis/vcv,alkadis/vcv,phihag/adhocracy,phihag/adhocracy,SysTheron/adhocracy,alkadis/vcv