The majority of the error checking can now be delegated to routines
like parse_f32(), parse_negative_i32() and so on, instead of creating
specific error messages for each type.
- use markdown instead of HTML, to make editing and translating easier
- use a shared prefix
- a few very minor wording tweaks
- we don't need to translate undocumented command errors
- share a string for positive number of days
- share a string for invalid property and state arguments, and avoid
listing them out
Related discussion: https://github.com/ankitects/anki/pull/922
