Skip items that aren't found by RelativePath
Skip items that aren't found by RelativePath

- Issue MatterHackers/MCCentral#2424
Cached thumbnails are invalid for zip items and containers

jlewin/MatterControl,mmoening/MatterControl,larsbrubaker/MatterControl,unlimitedbacon/MatterControl,mmoening/MatterControl,mmoening/MatterControl,unlimitedbacon/MatterControl,unlimitedbacon/MatterControl,unlimitedbacon/MatterControl,jlewin/MatterControl,jlewin/MatterControl,jlewin/MatterControl,larsbrubaker/MatterControl,larsbrubaker/MatterControl,larsbrubaker/MatterControl