builtin: Retry failed 304s by default

Named after vcl_backend_refresh from #3994.

bin/varnishd/builtin.vcl

@@ -257,9 +257,16 @@ sub vcl_backend_error {
 }
 
 sub vcl_builtin_backend_error {
+	call vcl_refresh_error;
 	call vcl_beresp_error;
 }
 
+sub vcl_refresh_error {
+	if (beresp.was_304) {
+		return (retry);
+	}
+}
+
 sub vcl_beresp_error {
 	set beresp.http.Content-Type = "text/html; charset=utf-8";
 	set beresp.http.Retry-After = "5";

doc/sphinx/reference/vcl_step.rst

@@ -392,6 +392,10 @@ circumstances, be cautious with putting private information there. If
 you really must, then you need to explicitly set ``beresp.ttl`` to
 zero in ``vcl_backend_error``.
 
+If a conditional fetch failed to process a 304 response and transitioned
+to ``vcl_backend_error``, the backend transaction is retried as a regular
+fetch.
+
 The `vcl_backend_error` subroutine may terminate with calling ``return()``
 with one of the following keywords:
 

doc/sphinx/reference/vcl_var.rst

@@ -1378,7 +1378,8 @@ beresp.was_304
 
 	When ``true`` this indicates that we got a 304 response
 	to our conditional fetch from the backend and turned
-	that into ``beresp.status = 200``
+	that into ``beresp.status = 200``, unless the refresh
+	attempt failed.
 
 
 obj