git-version-gen: include command name in one more diagnostic

git-version-gen: include command name in one more diagnostic

* build-aux/git-version-gen: When the required .tarball-version file
was missing or unreadable, you might see the diagnostic from "cat",
but no trace of the name of the invoking script.  Now, you still see
the diagnostic from cat, but also get one from "git-version-gen: ".
Inspired by a patch from Bruce Korb.