In Vim (and some vestigial parts of Nvim) E319 was a placeholder for
ex_ni commands, i.e. commands that are only available in certain builds
of Vim. That is obviously counter to Nvim's goals: all Nvim commands
are available on all platforms and build types (the remaining ex_ni
commands are actually just missing providers).
We need an error id for "missing provider", so it makes sense to use
E319 for that purpose.
ref #9344
ref #3577
61d0dfee32
