server: add hlx:proxyUrl meta tag also in case of 401 and 403 status

Is your feature request related to a problem? Please describe.
In order to load the sidekick on a 401/403 in the local proxy and allow users to log in, the DOM should contain a hlx:proxyUrl meta element.

Describe the solution you'd like
Injection of the meta tag is currently limited to status 200. This should be enhanced to support 401 and 403.