Wrong error response when uploading subtitles without having selected a language

[tfrommen]

When you upload a subtitle file without having selected a language, the error code 8 gets returned. The according message, however, is about already having subtitles for that language - which is not only wrong, but also just not possible.

Instead, the error code 7, indicating an invalid language, would be the right one to send.

Patch follows.

[obenland]

In 5855:

WP.tv: Use more appropriate error message.

In the unlikely case that no subtitle language was provided, let's bail saying
it's an invalid language rather than suggesting there is already a subtitle for
that language.

Props tfrommen.
Fixes #2958.