- Move the decision whether to translate an error message to where it is
defined. This simplifies call sites: no more `InitError(Untranslated(...))`.
- Make all functions in `util/error.h` consistently return a
`bilingual_str`. We've decided to use this as error message type so
let's roll with it.
This has no functional changes: no messages are changed, no new
translation messages are defined.
