Bad error message for invalid footer links in configuration

[LukasKalbertodt]

With this configuration:

[general]
footer_links = [
    { label = { de = "Foo" }, link = "https://opencast.org" },
]

Tobira emits this error:

 ‣ failed to read config file '../util/dev-config/config.toml'
   ‣ failed to deserialize configuration from file '../util/dev-config/config.toml'
     ‣ data did not match any variant of untagged enum FooterLink for key `general.footer_links` at line 50 column 1

That's not helpful at all. The problem here is that the language en is required, all other languages are optional. The error message should be improved.