Send back error tile for blocked sites

[grischard]

Currently, blocked sites get a 403 error, which doesn't get displayed.

We should communicate clearly to sites lacking attribution what the issue is and how they can fix it. In practice, they often don't know what they are doing wrong (did we run into rate limits?) and end up switching to another provider, still with no attribution.

For attribution blocks, we should configure fastly to respond with an uncacheable friendly error image. The HTTP error code needs to be 200 for the image to display in Leaflet.