This is some useful stuff from 4guysfromrolla.com. 

BEGIN TRY
   BEGIN TRANSACTION    -- Start the transaction

   -- Your code here

   -- If we reach here, success!
   COMMIT
END TRY
BEGIN CATCH
  -- Whoops, there was an error
  IF @@TRANCOUNT > 0
     ROLLBACK

  -- Raise an error with the details of the exception
  DECLARE @ErrMsg nvarchar(4000), @ErrSeverity int
  SELECT @ErrMsg = ERROR_MESSAGE(),
         @ErrSeverity = ERROR_SEVERITY()

  RAISERROR(@ErrMsg, @ErrSeverity, 1)
END CATCH