see: #16

I first though missing translations (typiaclly 3.7) should fail more cleanly than an exception logged with full stacktrace, probably something like "INFO: No translation for branch 3.7 in lang fr". But doing so I though we should probably just build them with the nearest translation, like 3.6, as lots of string are in common.

Tested locally with:

python3 ./build_docs.py --git --build-root /tmp/build_root/ --www-root /srv/docs.python.org --log-directory /tmp/log_root/ --skip-cache --quick