Added an error message to openmail (#6243)

Fixes #6159

Thanks to @SynchronyMaster

doc/script_commands.txt

@@ -5524,6 +5524,8 @@ Example:
 This will open a character's Mail window on the client connected to the
 invoking character.
 
+This command is not supported for PACKETVER 20150513 or newer.
+
 	mes "Close this window to open your mail inbox.";
 	close2;
 	openmail;

src/map/script.cpp

@@ -19435,14 +19435,20 @@ BUILDIN_FUNC(warpportal)
  **/
 BUILDIN_FUNC(openmail)
 {
-	TBL_PC* sd;
+	struct map_session_data* sd;
 
 	if (!script_charid2sd(2,sd))
 		return SCRIPT_CMD_FAILURE;
 
+#if PACKETVER < 20150513
 	mail_openmail(sd);
 
 	return SCRIPT_CMD_SUCCESS;
+#else
+	ShowError( "buildin_openmail: This command is not supported for PACKETVER 2015-05-13 or newer.\n" );
+
+	return SCRIPT_CMD_FAILURE;
+#endif
 }
 
 /**