ConvertCsvToAvro attempts to convert each record... if at least one record was converted successfully then it transfers that flow file to "success", if any records could not be converted then it also transfers a flow file to "incompatible" which has all of the original CSV in the content and a summary of the incompatible records in an attribute called "errors".
I think you need to implement a custom version of this processor that only transfers the flow file to success when the error count equals 0.