Warn on new json

In case someone moves translatable strings from .cpp to .json and doesn't add them to the extraction script. Thereafter, all .pot updates will automatically fail until someone resolves the situation.

Otherwise the strings get removed from the .pot file, which then trickles over to launchpad, which then removes all the translations because it thinks they're no longer necessary.

Note that it currently (correctly) fails because there is a text file full of things that should be translatable, but aren't, because they're in a magic-format text file.