add error message when supplied id is wrong

client/index.js

@@ -16,6 +16,10 @@ function interceptForm (formId) {
      * Param formId is the HTML id of the form
      */
     const formElem = document.getElementById(formId)
+    if (!formElem) {
+        console.error('You tried to intercept form id:"' + formId + '" but it was not found.')
+        return
+    }
     formElem.onsubmit = async (e) => {
         e.preventDefault()
         fetch(formElem.action, {