provider: repurpose E319

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

src/nvim/ex_cmds2.c

@@ -3830,7 +3830,8 @@ static void script_host_execute(char *name, exarg_T *eap)
     tv_list_append_number(args, (int)eap->line2);
 
     if (!eval_has_provider(name)) {
-      emsgf("No \"%s\" provider found. Run \":checkhealth provider\"", name);
+      emsgf("E319: No \"%s\" provider found. Run \":checkhealth provider\"",
+            name);
     } else {
       (void)eval_call_provider(name, "execute", args);
     }