Custom Error Message Handling?

Hi,
I am trying to create a custom error message and save it to the database.
Imagine, I have a file with the following fields A, B, C, D and corresponding database table columns (A,B,C,D,Comment)
Fields A & B are mandatory so inside the Validator I am using 'Is Null' function to validate A & B field.
If A is null I would like to insert a string "Invalid field A" in the column 'Comment' section. I still want to audit the whole rows but in the comment field I have a custom message telling that a certain field is invalid.
How do I accomplish this inside the Transformation - Validator?
see screen-shot of the actual record in the database that I want it to be.

You can use "If functions2 for this kind of transformation

For complete data quality audit you can use log redirect, default repository has an example

Mike