1. A method in a proxy server for responding to resource unavailable errors, comprising: receiving, from a client device, a request for a network resource hosted at an origin server for a domain, wherein the request is received at the proxy server as a result of a DNS (Domain Name System) request for the domain resolving to the proxy server, and wherein the origin server is one of a plurality of origin servers that belong to different domains that resolve to the proxy server and are owned by different entities; requesting the network resource from the origin server; receiving a response from the origin server that includes a resource unavailable error that indicates that the requested network resource is unavailable to the origin server; and responsive to receiving the response from the origin server that includes the resource unavailable error, the proxy server performing the following: creating a custom error page that indicates that the requested network resource is unavailable, wherein the custom error page is different than the received response from the origin server, and transmitting the custom error page to the client device that indicates that the requested resource is unavailable.