print warnings to stderr

[dustymabe]

$ ./fcct-x86_64-unknown-linux-gnu -version
Fedora CoreOS Config Transpiler v0.2.0
$ cat example.fcct | ./fcct-x86_64-unknown-linux-gnu > example.ign 
$ echo $?
0

Usually I would then take example.ign and feed it into an instance start, but it's actually invalid json because the warnings got put in the file:

$ cat example.ign | head -n 2
warning at $.systemd.units.1.name, line 41 col 13: unit "docker.mount" is enabled, but has no install section so enable does nothing
warning at $.systemd.units.2.name, line 49 col 13: unit "dockerdata.mount" is enabled, but has no install section so enable does nothing

Should we print those to stderr?