Restore customError property in the ValidityState() polyfill

Author: michapietsch

src/js/validityState-polyfill.js

@@ -93,6 +93,11 @@
 // Add valid property to validity object
 checkValidity.valid = valid;
 
+// Assume a custom error if there is a validation message present but all the other checks returned valid
+if (checkValidity.valid && field.validationMessage !== '') {
+checkValidity.customError = true
+}
+
 // Return object
 return checkValidity;