Handles failures better.

1. When 'failed' is closed !ok will be true -- we weren't properly
   hitting that case and the connection would stay open.
2. Cancels the request when a bad response code is hit.

main.go

@@ -111,16 +111,10 @@ func main() {
 		for {
 			chosen, value, ok := reflect.Select(cases)
 			if ok {
-				// Handle failure.
-				if value.Interface() == failed {
-					w.WriteHeader(500)
-					break
-				}
-
 				res := value.Interface().(*http.Response)
 
 				if allowedCodes[res.StatusCode] {
-					// Close all of the other pending request channels.
+					// Close all of the request channels.
 					for i := range cancels {
 						if i != chosen {
 							close(cancels[i])
@@ -131,10 +125,16 @@ func main() {
 						w.Header()[k] = v
 					}
 				} else {
+					close(cancels[chosen])
 					continue
 				}
 				io.Copy(w, res.Body)
 				break
+			} else {
+				// Handle failure.
+				w.WriteHeader(500)
+				w.(http.Flusher).Flush()
+				break
 			}
 		}
 	})