Report meaningful error when luerl is not available

src/prosody2ejabberd.erl

@@ -38,6 +38,8 @@
 %%% API
 %%%===================================================================
 from_dir(ProsodyDir) ->
+    case code:ensure_loaded(luerl) of
+	{module, _} ->
 	    case file:list_dir(ProsodyDir) of
 		{ok, HostDirs} ->
 		    lists:foreach(
@@ -56,6 +58,11 @@ from_dir(ProsodyDir) ->
 		    ?ERROR_MSG("failed to list ~s: ~s",
 			       [ProsodyDir, file:format_error(Why)]),
 		    Err
+	    end;
+	{error, _} = Err ->
+	    ?INFO_MSG("The file 'luerl.beam' is not found: maybe "
+		      "ejabberd is not compiled with Lua support", []),
+	    Err
     end.
 
 %%%===================================================================