Extend return values for some natives

[NexiusTailer]

There are some natives which always return 1/true, but they could have some arguments validity checks and start to return 0/false as well to indicate it did not execute with success. Those natives are:

• SendRconCommand/SendRconCommandf can check if the passed string is empty to return failure (just like it was made with GameTextForAll, for example)
• UnBlockIpAddress can also check if ip_address string is empty (just like BlockIpAddress do it now)
• SetGameModeText can return failure if the passed string is empty. Now it returns success, but gamemode text doesn't change and stays the previous set