fix(excmd): nlua_call_excmd require() failure is a "lua_error"

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.

test/functional/ex_cmds/excmd_spec.lua

@@ -8,7 +8,20 @@ local fn = n.fn
 local pcall_err = t.pcall_err
 local assert_alive = n.assert_alive
 
-describe('Ex cmds', function()
+describe('nlua_call_excmd excmds', function()
+  -- Exercise nlua_call_excmd by testing commands implemented with it (:log, :lsp).
+
+  before_each(function()
+    clear()
+  end)
+
+  it('error propagation, formatting', function()
+    t.eq('Vim(lsp):E5800: Invalid :lsp subcommand: bogus', pcall_err(command, 'lsp bogus'))
+    t.matches('Vim%(log%):E5200: No such log.*', pcall_err(command, 'log bogus'))
+  end)
+end)
+
+describe('excmds', function()
   before_each(function()
     clear()
   end)