Fix custom filter i18n failure

When a language establishes a convention of using plain
strings as exception types instead of something custom
that can hold e.g. a "code" property, it effectively waives its
right to i18n its error messages, or else programs cannot
differentiate between types of errors and catch/pcall
constructs are of very limited use.  Unfortunately it seems
that Lua did not realize this, and loadfile returns a plain
string that is not usable due to i18n.

Rather than trying to catch the "file not found" error, do
a scan to look for the file first, and if it's found in the
directory listing, assume any errors that happen on loading
it are fatal.

README.md

@@ -116,6 +116,6 @@ Example `customfilter.lua` that excludes all non-node items:
 ```lua
 return function(filtered, name, def)
 	if def.type ~= "node" then return false end
-	return filtered
+	return filtered -- or nil
 end
 ```

exportall.lua

@@ -3,8 +3,8 @@ local error, io, ipairs, loadfile, minetest, next, os, pairs, rawget,
       string, type
     = error, io, ipairs, loadfile, minetest, next, os, pairs, rawget,
       string, type
-local io_open, os_remove, string_gsub, string_match, string_sub
-    = io.open, os.remove, string.gsub, string.match, string.sub
+local io_open, os_remove, string_gsub, string_sub
+    = io.open, os.remove, string.gsub, string.sub
 -- LUALOCALS > ---------------------------------------------------------
 
 local include = ...
@@ -77,13 +77,18 @@ local function exportall()
 	outf:write("-- AUTOMATICALLY GENERATED by <https://gitlab.com/szest/defripper>"
 		.. "\n\nlocal reg = ...\n\n")
 
-	local custom, err = loadfile(outdir .. "/customfilter.lua")
-	if not custom then
-		if not string_match(err, "No such file or directory") then
-			return error(err)
+	local hascustomlua
+	for _, fn in pairs(minetest.get_dir_list(outdir, false)) do
+		if fn == "customfilter.lua" then
+			hascustomlua = true
+			break
 		end
-	else
-		custom = custom()
+	end
+	local custom
+	if hascustomlua then
+		local loaded, err = loadfile(outdir .. "/customfilter.lua")
+		if not loaded then return error(err) end
+		custom = loaded()
 		if type(custom) ~= "function" then
 			return error("customfilter.lua must return filter function")
 		end