image2qtree: Catch exceptions
image2qtree: Catch exceptions

Make it easy for the user to understand if they're trying to open a
non existant file or just ran out of memory.

DougFirErickson/visionworkbench,fengzhyuan/visionworkbench,AveRapina/visionworkbench,DougFirErickson/visionworkbench,DougFirErickson/visionworkbench,fengzhyuan/visionworkbench,AveRapina/visionworkbench,AveRapina/visionworkbench,DougFirErickson/visionworkbench,DougFirErickson/visionworkbench,fengzhyuan/visionworkbench,fengzhyuan/visionworkbench,DougFirErickson/visionworkbench,fengzhyuan/visionworkbench,AveRapina/visionworkbench,fengzhyuan/visionworkbench,AveRapina/visionworkbench,fengzhyuan/visionworkbench,AveRapina/visionworkbench