Handle Error Question

I have the next sp:

CREATE PROCEDURE SP_SALDO
(
@strDatos char(50)
)

AS


declare @ID char(20)
declare @Montodecimal(9,2)
declare @SaldoActualdecimal(9,2)
DECLARE @TipMov tinyint
declare @Fechachar(11)
declare @Horachar(10)
declare @Tiendatinyint
declare @NoCajatinyint
declare @SaldoNuevo decimal(9,2)
declare @NoAutoint
DECLARE @NomEmp char(40)
DECLARE @Trans_Num numeric

set nocount on

--SET @strDatos='T23C02001000000000000000000020025047'

SET @ID= SUBSTRING(@strDatos , 10, 20 )
set @Monto= cast(SUBSTRING(@strDatos , 30, 7 ) as decimal(9,2))/100
set @TipMov= cast(SUBSTRING(@strDatos , 7, 3 ) as int)
set @Tienda= cast(SUBSTRING(@strDatos , 2, 2 ) as int)
set @NoCaja= cast(SUBSTRING(@strDatos , 5, 2 ) as int)
set @Fecha = convert(char(11), getdate(), 101)
set @Hora= CONVERT(VARCHAR,getdate(),108)
set @NoAuto=ABS(CHECKSUM(NewID())) % 100000 + 1


--Se guarda el monto Actual y el nombre en variables
SElect @SaldoActual = SALDO, @NomEmp = NOM_EMP, @Trans_Num=last_trans_num +1
FROM TBSALDOS
WHERE ID=@ID
--select @SaldoActual,@ID,@Monto,@TipMov,@Tienda, @NoCaja, @Fecha, @Hora,@NoAuto

if (@Monto<=@SaldoActual)
begin
set @SaldoNuevo=@SaldoActual-@Monto

insert TBMOVIMIENTOS
values(@trans_num,@ID,@TipMov,GETDATE(),@hora,@Tienda,@NoCaja,@SaldoActual,@Monto,@NoAuto)

Update TBSALDOS
SET SALDO=@SaldoNuevo, SALDO_ULT_MOV=@Monto,
DT_ULT_MOV=GETDATE(), HR_ULT_MOV=@Hora,
NO_TIE_ULT_MOV=@Tienda, NO_CAJA_ULT_MOV=@NoCaja,
last_trans_num=@Trans_Num
WHERE ID=@ID

SELECT 'OK'+ SUBSTRING(@strDatos , 1, 6 )+@id
+RIGHT(REPLICATE('0',7) + CAST(cast(@SaldoNuevo*100 as int) as varchar(10)) ,7)
+ RIGHT(REPLICATE(' ',40) +@NomEmp ,40)
+ + RIGHT(REPLICATE('0',6) + CAST(cast(@NoAuto as int) as varchar(10)) ,6) as resp
end
else
begin
SELECT 'NO'+ SUBSTRING(@strDatos , 1, 6 )+@id
+RIGHT(REPLICATE('0',7) + CAST(cast(@SaldoActual*100 as int) as varchar(10)) ,7)
+ RIGHT(REPLICATE(' ',40) +@NomEmp ,40)
+ '000000' as resp
end
GO

How can I add a handle error?

What I need to do is if you can not do the insert and the update returns :

SELECT 'NO'+ SUBSTRING(@strDatos , 1, 6 )+@id
+RIGHT(REPLICATE('0',7) + CAST(cast(@SaldoActual*100 as int) as varchar(10)) ,7)
+ RIGHT(REPLICATE(' ',40) +@NomEmp ,40)
+ '000000' as resp
end