Although `nlua_call_excmd` is semantically for implementing Ex-commands,
the `require()` should never fail, so that's a "Lua error".
But if the call itself fails (the later `semsg` call), that's an "Ex
cmd" error.
Problem:
Too much boilerplate needed to use Lua to impl an excmd or f_xx
function.
Solution:
- Add `nlua_call_vimfn` which takes the args typval, executes
Lua, and returns a typval.
- refactor(excmd): lua impl for :log, :lsp
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.
