More clearly abort refused connections downstream

Mildly more accurate proxying I think (though less technically correct)
but especially useful because its the error we receive for failed proxy
connections, which will imminently become relevant.

src/rules/requests/request-handlers.ts

@@ -929,7 +929,7 @@ export class PassThroughHandler extends PassThroughHandlerDefinition {
                 }
                 clientRes.tags.push('passthrough-error:' + e.code);
 
-                if (e.code === 'ECONNRESET') {
+                if (e.code === 'ECONNRESET' || e.code === 'ECONNREFUSED') {
                     // The upstream socket closed: forcibly close the downstream stream to match
                     const socket: net.Socket = (clientReq as any).socket;
                     socket.destroy();

test/integration/subscriptions/response-events.spec.ts

@@ -467,7 +467,8 @@ describe("Abort subscriptions", () => {
                 await server.on('response', (r) => seenResponsePromise.resolve(r));
 
                 await server.forAnyRequest().thenPassThrough({
-                    proxyConfig: { proxyUrl: `http://localhost:8901` }
+                    // Wrong port: this connection will fail
+                    proxyConfig: { proxyUrl: `http://localhost:8999` }
                 });
 
                 fetch(server.urlFor("/mocked-endpoint")).catch(() => {});