Hi,

this patch is based on our discussion on the IRC channel from today.

Basically, when abipkgdiff is given a non-existent path for a package, 
the logic will let it pass and just mark it as an unknown kind of 
package, giving the vague warning "<PKG> should be a valid package 
file". This patch ensures we properly diagnose the issue and quit 
prematurely.

Cheers,

Ondrej