Post in multiple languages, but not in DEFAULT_LANG, leads to error

[felixfontein]

Environment (if reporting a bug)

Nikola Version: master

Description:

If I follow the instructions in #2956, but enter languages en,es,de, and copy posts/test.es.rst also to posts/test.de.rst, I get another error (or rather, a bunch of them):

[2018-05-04T20:24:51Z] ERROR: Nikola: Two posts are trying to generate posts/test/index.html: posts/test.es.rst and posts/test.de.rst
[2018-05-04T20:24:51Z] ERROR: Nikola: Two posts are trying to generate posts/test/index.rst: posts/test.es.rst and posts/test.de.rst
[2018-05-04T20:24:51Z] ERROR: Nikola: Two posts are trying to generate es/posts/test/index.html: posts/test.es.rst and posts/test.de.rst
[2018-05-04T20:24:51Z] ERROR: Nikola: Two posts are trying to generate es/posts/test/index.rst: posts/test.es.rst and posts/test.de.rst
[2018-05-04T20:24:51Z] ERROR: Nikola: Two posts are trying to generate de/posts/test/index.html: posts/test.es.rst and posts/test.de.rst
[2018-05-04T20:24:51Z] ERROR: Nikola: Two posts are trying to generate de/posts/test/index.rst: posts/test.es.rst and posts/test.de.rst

[felixfontein]

I added a fix for this to #3072.