[Improvement™]

I use this methode already for about 1-2 years, never had any kind of problems with this.
Perhaps this is what you are searching for, I also added your desired message.

pawn Код:

public OnPlayerCommandText(playerid, cmdtext[])
}
    return SendClientMessage(playerid, COLOR_RED, "No such command! Type /cmds for a list of player commands.");
}