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.

scripts/linterrcodes.lua

@@ -48,7 +48,7 @@ local dup_allowed = {
   E509 = 2,
   E5101 = 2,
   E5102 = 2,
-  E5108 = 6,
+  E5108 = 5,
   E5111 = 2,
   E513 = 2,
   E521 = 2,