Handle file not existing in Stagecraft linter
Fix client-side error (D3 not loaded) in old IE

In 8ae84d292ee4b40a8ea046046da2c578713c17f4, d3 was introduced into config
objects to improve axis ticks.

It works fine in good browsers, but in at least IE 8 it causes a script
error. This means that even though we don't use this JavaScript, the JS
error causes us not to render the fallback PNG images.
Merge pull request #264 from alphagov/67110874-ie8-png-fallback-fix

Fix fallback images in IE 8