ZIP Import & Exports: Addressed issues during testing

- Handled links to within-zip page images found in chapter/book
  descriptions; Added test to cover.
- Fixed session showing unrelated success on failed import.

Tested import file-create undo on failure as part of this testing.

app/Exports/Controllers/ImportController.php

@@ -89,6 +89,8 @@ class ImportController extends Controller
         try {
             $entity = $this->imports->runImport($import, $parent);
         } catch (ZipImportException $exception) {
+            session()->flush();
+            $this->showErrorNotification(trans('errors.import_zip_failed_notification'));
             return redirect($import->getUrl())->with('import_errors', $exception->errors);
         }