Add header controls for CDN in error cases

[ruflin]

Our CDN supports headers on what to do in case of errors. These are:

Surrogate-Control: max-age=3600, stale-if-error=86400

In case the registry is down or for example during upgrades, it can be useful that the CDN still ships the stale data. For this, it the package-registry should add these headers to the different endpoints.

Here are some docs: https://developer.fastly.com/reference/vcl/variables/miscellaneous/beresp-stale-if-error/