Fix for issue 434

Issue https://github.com/mozilla/marketplace-tests/issues/434
As you can see in the screenshot the test does not wait if images are in preview-error state.
Let us see if this helps. 
I will start and adhoc job.
Screenshot: http://qa-selenium.mv.mozilla.com:8080/view/Marketplace/job/marketplace.stage.developer_hub.saucelabs/620/HTML_Report/debug/tests_desktop_developer_hub_test_developer_hub_TestDeveloperHub/test_that_a_screenshot_can_be_added/screenshot.png
Adhoc job: http://qa-selenium.mv.mozilla.com:8080/view/Marketplace/job/marketplace.stage.developer_hub.saucelabs.fix/