--iso with invalid pathname -> silently burns whatever iso it finds in ~/Downloads

[James Ferguson]

Binary package hint: usb-creator-gtk

Ubuntu 10.10
usb-creator-gtk 0.2.26

Use command line:

    usb-creator-gtk -i <invalid-pathname.iso>

And all appears well, but what actually occurs is it burns whatever iso it can find in ~/Downloads.

This has bitten me quite a bit in a couple of scenarios, including executing it in the wrong directory with a relative path for the iso.

Expected Beharior: non-readable --iso pathname should cause the program to exit

I think this is a serious problem.

I think the source of the issue is that Backend.add_image() silently returns if the pathname is not valid.