Validate Spreadsheet in a HTML Service

I have an html form I created with HTML Service in Google Sheets. The form has three fields in it. I want to check if the data they entered in one of the fields is contained in a column of sheet2. If it is not, I just want to display an alert box saying "invalid input". Whats the best way to do this? This is my submit form. The line of code that compares the value to "test", is where I want to check against a value in the spreadsheet.

function formSubmit() {

  if (document.getElementById("sku").value === "test") {

    alert(document.getElementById("sku").value);
  } else {
    google.script.run.appendRowstoSheet(document.forms[0]);
    document.getElementById("sku").value = ' ';
    document.getElementById("sku").focus();
  }
}

gs Code:

//insertValuestoSheet
function appendRowstoSheet(form){
  var sku = form.sku,
  loc_array = [{}];
  location = form.location,
  reference = form.reference

  loc_array = location.split("-");
  sheet = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet();
  //validateMySheet();

  //don't insert blank sku's
  //Also want to have it so it doesn't insert values where if the sku
  //doesn't match a sku in sheet2, column A. Alert the user if this condition

  if (sku != " ") {
    sheet.appendRow(["'"+reference," "," "," ","'"+sku, "1", " ", " ", " ", "'"+loc_array[0],"'"+loc_array[1],"'" + loc_array[2],"'"+loc_array[3]]);
  }
}

I modified my code with a better function name and added the function. Currently in appendRowstoSheet, it inserts into the spreadsheet. How do I return a failure here if my condition isn't met. Am I understanding that correctly?