assert when the upload helper can't find the selector

also, fix the assertion so it uses normal syntax instead
of ember-cli-page-object `findElement` syntax

test-support/helpers/upload.js

@@ -1,7 +1,7 @@
 /*global triggerEvent, find */
 
 export default function (selector, file, filename) {
-  let input = find(this, selector)[0];
+  let input = findWithAssert(selector)[0];
 
   file.name = filename;