fix: loading error pages after building shared config

[StokesAsselborn]

To avoid going touching the SharedConfig struct and modifying the handlers etc, we are now doing the following :

1. build_shared_config : populates std::map<HttpResponse::Status, std::string> error_pages where key = status and value = URI
2. before returning the route in build_route_config(), we attempt load the error pages. On success, the URI is replaced by the HTML. On failure, the URI is removed to let the HttpResponse to use the default error pages.

No need to mention that this is no good practice and that we should rather use to distinct maps or a small struct for each error page. We can log this as enhancement.

[StokesAsselborn]

To test :

curl -i http://localhost:8080/does-not-exist
curl -i http://localhost:8080/override/
curl -i http://localhost:8080/nooverride/