Pipe response from SSaaS instead of using event handlers

One line of code that does the exact same thing is better than 6.
Add "nice" error pages for SSaaS failures

SSaaS now returns errors with appropriate status codes when requesting a page to screenshot fails. We can use these to render nice errors in spotlight when following broken PNG links.
Merge pull request #754 from alphagov/bugfix/ssaas

Render "nice" error pages for failed SSaaS requests