Idea for error codes.

errors.pwn

#define ERROR_COUNT // NO VALUE, so it merely exists.

enum Error: // Generic function errors.
{
	OK = 0,
	ERR_NONE = 0,
	ERR_PARAM_0,
	ERR_PARAM_1,
	ERR_PARAM_2,
	ERR_PARAM_3,
	ERR_PARAM_4,
	ERR_PARAM_5,
	ERR_PARAM_6,
	ERR_PARAM_7,
	ERR_PARAM_8,
	ERR_PARAM_9,
	ERR_PARAM_10,
	ERR_PARAM_11,
	ERR_PARAM_12,
	ERR_PARAM_13,
	ERR_PARAM_14,
	ERR_PARAM_15,
	ERR_PARAM_16,
	ERR_PARAM_17,
	ERR_PARAM_18,
	ERR_PARAM_19,
	ERR_PARAM_20,
	ERR_PARAM_21,
	ERR_PARAM_22,
	ERR_PARAM_23,
	ERR_PARAM_24,
	ERR_PARAM_25,
	ERR_PARAM_26,
	ERR_PARAM_27,
	ERR_PARAM_28,
	ERR_PARAM_29,
	ERR_PARAM_30,
	ERR_PARAM_31,
	ERR_OVERFLOW,
	ERR_UNDERFLOW,
	_ERR_PARAMS_DONE,
}

#undef ERROR_COUNT
#define ERROR_COUNT (_ERR_PARAMS_DONE)

enum Error: // Player errors.
{
	ERR_PLAYER_NOT_CONNECTED = ERROR_COUNT,
	ERR_PLAYER_CONNECTED,
	ERR_PLAYER_INSUFFICIENT_FUNDS,
	ERR_PLAYER_DEAD,
	ERR_PLAYER_SPAWNED,
	ERR_PLAYER_IN_VEHICLE,
	ERR_PLAYER_NOT_IN_VEHICLE,
	ERR_PLAYER_NO_WEAPON,
	_ERR_PLAYER_1_DONE,
}

#undef ERROR_COUNT
#define ERROR_COUNT (_ERR_PLAYER_1_DONE)

// Use:
//
//   return Error:-2;
//
// For custom errors.

Error:GivePlayerMoneyClamped(playerid, money, min = 0, max = cellmax, bool:approx = false)
{
	if (!IsPlayerConnected(playerid)) return ERR_PLAYER_NOT_CONNECTED;
	new cur = GetPlayerMoney(playerid), diff;
	if (money < 0)
	{
		diff = min - cur;
		if (money < diff)
		{
			if (approx)
			{
				GivePlayerMoney(playerid, diff);
				return OK;
			}
			return ERR_OVERFLOW;
		}
	}
	else
	{
		diff = max - cur;
		if (diff < money)
		{
			if (approx)
			{
				GivePlayerMoney(playerid, diff);
				return OK;
			}
			return ERR_OVERFLOW;
		}
	}
	GivePlayerMoney(playerid, money);
	return OK;
}