Since deploying Termbox to production there have been a few requests producing 404s on the Termbox SSR service due to validation errors and invalid language errors. These languages are likely produced by language headers set by users' browsers.

Instead of letting the termbox service fail we could handle these errors on the Wikibase end, thereby preventing requests that are known to not produce valid responses.

Language producing validation error: https://logstash.wikimedia.org/app/kibana#/doc/logstash-*/logstash-2019.08.22/wikibase-termbox?id=AWy5qmCKCfBFUvHRtPvB
Unknown content language: https://logstash.wikimedia.org/app/kibana#/doc/logstash-*/logstash-2019.08.22/wikibase-termbox/?id=AWy4MJsEZKA7Rpir06bd

In such cases there is no usable response from the service.

Question: Should it fall back to english like desktop termbox does?