function showMessage(message, caller) {

  // Sets the title using the APP_TITLE variable; adds optional caller string.
  const title = APP_TITLE
  if (caller != null) {
    title += ` : ${caller}`
  };

  const ui = SpreadsheetApp.getUi();
  ui.alert(title, message, ui.ButtonSet.OK);
}
A helper function to display messages to user.

@param {string} message - Message to be displayed.
@param {string} caller - {Optional} text to append to title.