efsm: don't produce anything when we don't support the automaton type.
We must not emit anything before spitting the error:
$ vcsn standard -Clal,q -Oefsm -Ee a+a
#! /bin/sh
me=$(basename "$0")
medir=$(mktemp -d "/tmp/$me.XXXXXX") || exit 1
arc_type=standard: invalid weightset: q, expected: b log nmin rmin zmin
We should check first, and then process.