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

test/functional/provider/python_spec.lua

@@ -8,6 +8,7 @@ local funcs = helpers.funcs
 local meths = helpers.meths
 local insert = helpers.insert
 local expect = helpers.expect
+local expect_err = helpers.expect_err
 local command = helpers.command
 local exc_exec = helpers.exc_exec
 local write_file = helpers.write_file
@@ -17,7 +18,11 @@ local missing_provider = helpers.missing_provider
 do
   clear()
   if missing_provider('python') then
-    pending('Python 2 (or the neovim module) is broken/missing', function() end)
+    it(':python reports E319 if provider is missing', function()
+      expect_err([[Vim%(python%):E319: No "python" provider found.*]],
+                 command, 'python print("foo")')
+    end)
+    pending('Python 2 (or the pynvim module) is broken/missing', function() end)
     return
   end
 end