--appimage-extract <non-existant-file> exits 0 and no error message

I would expect that trying to extract a file which doesn't exist in the image would exit non-0.

` --appimage-extract `

What I get is a null output and a 0 exit code (which by convention means that everything worked OK)

The exit code can be viewed with a POSIX compatible shell via: `echo $?`

Likewise I expect a human-readable error message when any of the requested files don't exist in the image.