The only message it shows, even when verbose=true is:
[deb] Creating debian package: /dir/packg.deb
[deb] Building data
[deb] Failed to create the Debian package /dir/packg.deb
BUILD FAILED
/dir/build.xml:80: Failed to create the Debian package /dir/packg.deb
I've seen this answer but it only applies to Eclipse users.
If Eclipse has it, this file must be saved somewhere.
You can make ant be extra verbose by passing it the "-v" parameter. This will show the stack trace in the console.