Problem:
- Choosing a new EXX error code is tedious.
- It's possible to accidentally use an EXX error code for different
purposes.
Solution:
Add a lint check which requires EXX error codes to have a :help tag.
This also avoids duplicates because `make doc` does `:helptags ++t doc`
which fails if duplicates are found.
(cherry picked from commit bc6d946cca)
