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Sounds like you need to catch your error and allow the code to continue.
off for lunch but could discuss further after if you need.

if I catch it and allow it to save, then it won't return the csv response with the line error

Unless there is another way to add an error message for the csv response?

In my experience, the error from the User Event script should not prevent the CSV Import wizard from processing the other records in the file.  The Import report will only show the thrown error in the relevant record.