fix(router): fix change in behaviour on load error occur

Clear the stored loader promise so that subsequent load will try the fetch again.
The restores the same behaviour as before we cached the loader to fix  angular#26557

packages/router/src/router_config_loader.ts

@@ -8,7 +8,7 @@
 
 import {Compiler, InjectionToken, Injector, NgModuleFactory, NgModuleFactoryLoader} from '@angular/core';
 import {from, Observable, of} from 'rxjs';
-import {map, mergeMap, share, tap} from 'rxjs/operators';
+import {catchError, map, mergeMap, share} from 'rxjs/operators';
 
 import {LoadChildren, LoadedRouterConfig, Route} from './config';
 import {flatten, wrapIntoObservable} from './utils/collection';
@@ -36,6 +36,10 @@ export class RouterConfigLoader {
       const moduleFactory$ = this.loadModuleFactory(route.loadChildren!);
 
       route._loader$ = moduleFactory$.pipe(
+          catchError((err) => {
+            route._loader$ = undefined;
+            throw err;
+          }),
           map((factory: NgModuleFactory<any>) => {
             if (this.onLoadEndListener) {
               this.onLoadEndListener(route);