Fail loading the 3MF file if no meshes were found
Fail loading the 3MF file if no meshes were found

This way we indicate loading the 3MF file failed instead of indicating
success but not actually loading anything.

CURA-236

ad1217/Cura,ynotstartups/Wanhao,fieldOfView/Cura,bq/Ultimaker-Cura,hmflash/Cura,fieldOfView/Cura,Curahelper/Cura,markwal/Cura,bq/Ultimaker-Cura,senttech/Cura,ynotstartups/Wanhao,totalretribution/Cura,ad1217/Cura,hmflash/Cura,totalretribution/Cura,senttech/Cura,markwal/Cura,Curahelper/Cura