Error messages
Error messages are a central part of the grammarchecker. Each error
tag (the ones starting with &) must have one and only one error message, but
the same error message may be used for several error tags.
The tags are written in the files errors-en.ftl and errors-xxx.ftl
(xxx being the ISO 639-1 (if it exists) or ISO 639-3 code for the
target language).
Each tag must be declared in the errors.json file (tbw: one-many corr).