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

runtime/doc/message.txt

@@ -514,15 +514,6 @@ command.  The script can then again read another script.  This can continue
 for about 14 levels.  When more nesting is done, Vim assumes that there is a
 recursive loop somewhere and stops with this error message.
 
-							*E319*  >
-  The command is not available in this version
-
-You have used a command that is not present in the version of Vim you are
-using.  When compiling Vim, many different features can be enabled or
-disabled.  This depends on how big Vim has chosen to be and the operating
-system.  See |+feature-list| for when which feature is available.  The
-|:version| command shows which feature Vim was compiled with.
-
 							*E300*  >
   Swap file already exists (symlink attack?)
 

runtime/doc/provider.txt

@@ -6,7 +6,14 @@
 
 Providers	 				*provider*
 
-Nvim delegates some features to dynamic "providers".
+Nvim delegates some features to dynamic "providers". This document describes
+the providers and how to install them.
+						*E319*
+Use of a feature requiring a missing provider is an error: >
+
+    E319: No "foo" provider found. Run ":checkhealth provider"
+
+Run the |:checkhealth| command, and review the sections below.
 
 				      Type |gO| to see the table of contents.
 

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);
     }

test/functional/provider/python3_spec.lua

@@ -2,6 +2,7 @@ local helpers = require('test.functional.helpers')(after_each)
 local eval, command, feed = helpers.eval, helpers.command, helpers.feed
 local eq, clear, insert = helpers.eq, helpers.clear, helpers.insert
 local expect, write_file = helpers.expect, helpers.write_file
+local expect_err = helpers.expect_err
 local feed_command = helpers.feed_command
 local source = helpers.source
 local missing_provider = helpers.missing_provider
@@ -9,7 +10,11 @@ local missing_provider = helpers.missing_provider
 do
   clear()
   if missing_provider('python3') then
-    pending('Python 3 (or the neovim module) is broken/missing', function() end)
+    it(':python3 reports E319 if provider is missing', function()
+      expect_err([[Vim%(python3%):E319: No "python3" provider found.*]],
+                 command, 'python3 print("foo")')
+    end)
+    pending('Python 3 (or the pynvim module) is broken/missing', function() end)
     return
   end
 end

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