Debug stored procedure

[stockton]

I have the following Stored Procedure which works for one client but not another:-

ALTER PROCEDURE spIssueScannedTickets

        @iEventID int,

        @MemberNum nvarchar(12),

        @BatchSize int,

        @FirstNumber nvarchar(12),

        @LastNumber nvarchar(12),

        @SlotsBalance int,

        @TableBalance int,

        @BonusBalance int,

        @UserID int

AS

DECLARE @Result int

SET @Result = 0

BEGIN TRANSACTION

if  (@SlotsBalance>0)

INSERT INTO TicketsIssued (Date, EventID, MemberNumber, EarnType, NumTicketsIssued, Turnover, UserID)

         VALUES (GETDATE(),@iEventID,@MemberNum, 'Slots', @SlotsBalance, 0, @UserID)

SELECT @Result = @@error IF @Result <> 0 BEGIN ROLLBACK TRANSACTION RETURN @Result END

if  (@TableBalance>0)

INSERT INTO TicketsIssued (Date, EventID, MemberNumber, EarnType, NumTicketsIssued, Turnover, UserID)

         VALUES (GETDATE(),@iEventID,@MemberNum, 'Tables', @TableBalance, 0, @UserID)

SELECT @Result = @@error IF @Result <> 0 BEGIN ROLLBACK TRANSACTION RETURN @Result END

if  (@BonusBalance>0)

INSERT INTO TicketsIssued (Date, EventID, MemberNumber, EarnType, NumTicketsIssued, Turnover, UserID)

         VALUES (GETDATE(),@iEventID,@MemberNum, 'Bonus', @BonusBalance, 0, @UserID)

SELECT @Result = @@error IF @Result <> 0 BEGIN ROLLBACK TRANSACTION RETURN @Result END

INSERT INTO BundlesIssued (MemberNumber, EventID, BundleSize, FirstNumber, LastNumber, DateIssued, UserID, Invalid)

        VALUES (@MemberNum, @iEventID, @BatchSize, @FirstNumber, @LastNumber, GETDATE(), @UserID , 0x00)

SELECT @Result = @@error IF @Result <> 0 BEGIN ROLLBACK TRANSACTION RETURN @Result END

COMMIT TRANSACTION

SELECT @Result = @@error IF @Result <> 0 RETURN @Result

SET @Result = @BatchSize RETURN @Result



GO

I need it to return something better than a -1 when it fails.
How do I go about getting decent error messages from the Stored Procedure?