LibJS+Everywhere: Propagate Cell::initialize errors from Heap::allocate

Callers that are already in a fallible context will now TRY to allocate
cells. Callers in infallible contexts get a FIXME.

Userland/Libraries/LibWeb/DOM/NodeIterator.cpp

@@ -53,7 +53,7 @@ JS::NonnullGCPtr<NodeIterator> NodeIterator::create(Node& root, unsigned what_to
     // 2. Set iterator’s root and iterator’s reference to root.
     // 3. Set iterator’s pointer before reference to true.
     auto& realm = root.realm();
-    auto iterator = realm.heap().allocate<NodeIterator>(realm, root);
+    auto iterator = realm.heap().allocate<NodeIterator>(realm, root).release_allocated_value_but_fixme_should_propagate_errors();
 
     // 4. Set iterator’s whatToShow to whatToShow.
     iterator->m_what_to_show = what_to_show;