How to display the error message in Sales Order while scheduling using the standard MSC_ATP_CUSTOM.C

I am using MSC_ATP_CUSTOM.CUSTOM_POST_ATP_API hook to derive the  SSD based on the  business requirement and we have to raise exception when certain conditions are satisfied? Question is :How to display the error message in Sales Order while scheduling using the standard MSC_ATP_CUSTOM.CUSTOM_POST_ATP_API?

I am populating the x_return_status variables as 'E', whenever there is an exception, but still the exception is not getting raised and stopping the USER from proceeding further.

Please suggest