Error message using User Event

The following user event script throws error for Unique value fields if duplicate values is entered.
The script will throw error for record create/edit in UI and also via SOAP API.
The fields are :BBCPRODUCTID_AU,BBCPRODUCTID_MY

/**
 * @NApiVersion 2.1
 * @NScriptType UserEventScript
 */

define(["N/record", "N/search", "N/error"],

  (record, search, error) => {

    /**
      * @description search to find if the value in the field already exist
      * @param {fieldId} fieldId of the item to be searched
      * @param {value} current value in the field
      */
    function itemFieldValueSearch(fieldId, value) {
      try {
        let itemSearchObj = search.create({
          type: "item",
          filters:
            [fieldId, "is", value]
          ,
          columns:
            [
              search.createColumn({ name: "internalid", label: "Internal ID" })
            ]
        });
        let searchResultCount = itemSearchObj.runPaged().count;
        return searchResultCount;
      } catch (e) {
        log.error("error @ itemFieldValueSearch ", e);
      }
    }

    /**
     * Defines the function definition that is executed before record is submitted.
     * @param {Object} scriptContext
     * @param {Record} scriptContext.newRecord - New record
     * @param {Record} scriptContext.oldRecord - Old record
     * @param {string} scriptContext.type - Trigger type; use values from the context.UserEventType enum
     * @since 2015.2
     */
    const beforeSubmit = (scriptContext) => {
      // Validation on Create context
      if (scriptContext.type == 'create') {
        let errMsgFields = [];

        try {
          let newRec = scriptContext.newRecord;
          // Validation check for field : BBCPRODUCTID_AU
          let productIdAU = newRec.getValue({ fieldId: 'custitemcustitem_jj_bggn_1081' });
          if (productIdAU) {
            countAU = itemFieldValueSearch("custitemcustitem_jj_bggn_1081", productIdAU);
            if (countAU > 0) {
              errMsgFields.push("BBCPRODUCTID_AU");
            }
          }

          // Validation check for field : BBCPRODUCTID_MY
          let productIdMY = newRec.getValue({ fieldId: 'custitem_jj_bggn_1082' });
          if (productIdMY) {
            countAU = itemFieldValueSearch("custitem_jj_bggn_1082", productIdMY);
            if (countAU > 0) {
              errMsgFields.push("BBCPRODUCTID_MY");
            }
          }

        // if validation fails throw error message
        if (errMsgFields.length > 0) {
          // create error msg string
          errorMsg = errMsgFields.join(", ");
          let msgString = (errMsgFields.length == 1) ? `Uniqueness error for Field: ${errorMsg}` : `Uniqueness error for Fields: ${errorMsg}`;
          throw msgString;
        }
      }

    }
    return { beforeSubmit }

  });