Error message cleanup

[glenrobson]

Fixes: #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 Error message cleanup #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 2662e3c on error_message_cleanup into 97625e1 on master.

[zimeon]

Looks sensible