Log an error when slicing failed
Log an error when slicing failed

In the command socket, when slicing failed, it would just ignore the failure and continue. This is fine, but let's at least log an error when that happens.
Ultimaker/CuraEngine,alephobjects/CuraEngine,totalretribution/CuraEngine,ROBO3D/CuraEngine,pratikshashroff/pcura,markwal/CuraEngine,Ultimaker/CuraEngine,ROBO3D/CuraEngine,alephobjects/CuraEngine,pratikshashroff/pcura,pratikshashroff/pcura,alephobjects/CuraEngine,totalretribution/CuraEngine,ROBO3D/CuraEngine,markwal/CuraEngine,markwal/CuraEngine,totalretribution/CuraEngine