Internationalize can't-parse-server-response error messages

[speezepearson]

Normally, the client simply displays the pre-i18ned messages that the server sends back. But what about when the client can't parse the server's response? It would need the appropriate i18n of "can't parse server response" baked into it during some early page-load. (@vjkoskela suggests making index.html a Twirl template, but there are surely other ways too.)