Error message cleanup

[glenrobson]

Fixes: https://github.com/IIIF/image-validator/issues/63

Currently AWS sends a email of there are a high percentage of non 200 requests. This means I get notified regularly when bots try and crawl the sites for files that don't exist. The image validator currently returns 500 if it receives a POST for a page that doesn't exist. This pull request does the following:

• Return 404 for a page that doesn't exist
• Hide common requests from the apache log for files that don't exist (404 is still returned). This should stop the notifications for known files that don't exist (see #64 for a list).
• Redirects http://image-validator.iiif.io/ to https://iiif.io/api/image/validator/ as there is no page at the route of this webapp.

This should mean notifications are only sent if there is a genuine problem with the service.

[coveralls]

Coverage decreased (-0.05%) to 33.424% when pulling 2662e3c695d7d744899d6dd4bc6a9d908ff18570 on error_message_cleanup into 97625e17ffecdac8e2338773ebfbf3399c625170 on master.