Add 404.png used by sites using old OpenLayers example

[Firefishy]

Add a message in the old OpenLayer 404.png

[Firefishy]

We respond with around 8 million 410 - GONE responses for this 404.png tiles a day. Having the file available and responding with cache headers will hopefully reduce the number of requests.

[tomhughes]

The requests don't really cost anything, and this certainly doesn't belong here - it's an operational issue for osm.org not an issue for this repo.

[Firefishy]

8 million requests a day / 100 requests a second (>32 million fd stats and >16 million fs reads) does cost and is not insignificant. We already have legacy OpenLayers files in this repo, adding 1 static file will mostly get rid of the OpenLayers DDOS failure request loop that has been hitting us for years.

Would you prefer the 404.png file was dropped by chef?

[Firefishy]

~2.8GB of access.log logs per day just to store the 410 request responses.

[Firefishy]

This PR is now made redundant by updated to openstreetmap/chef#325