feat(endpoints): enhance offline browsing (#2454)

* feat(api): rewrite error response when trying to query a down endpoint

* feat(interceptors): adding custom backend return code on offline fastfail

api/http/handler/endpointproxy/proxy_docker.go

@@ -1,6 +1,7 @@
 package endpointproxy
 
 import (
+	"errors"
 	"strconv"
 
 	httperror "github.com/portainer/libhttp/error"
@@ -23,6 +24,10 @@ func (handler *Handler) proxyRequestsToDockerAPI(w http.ResponseWriter, r *http.
 		return &httperror.HandlerError{http.StatusInternalServerError, "Unable to find an endpoint with the specified identifier inside the database", err}
 	}
 
+	if endpoint.Status == portainer.EndpointStatusDown {
+		return &httperror.HandlerError{http.StatusServiceUnavailable, "Unable to query endpoint", errors.New("Endpoint is down")}
+	}
+
 	err = handler.requestBouncer.EndpointAccess(r, endpoint)
 	if err != nil {
 		return &httperror.HandlerError{http.StatusForbidden, "Permission denied to access endpoint", portainer.ErrEndpointAccessDenied}