fix(router): return promise with error handler

See https://github.com/angular/angular/pull/2528\#discussion_r32493195

Author: btford

modules/angular2/src/router/router.ts

@@ -128,12 +128,10 @@ export class Router {
  ObservableWrapper.callNext(this._subject, matchedInstruction.accumulatedUrl);
  });
 
- PromiseWrapper.catchError(result, (err) => {
+ return PromiseWrapper.catchError(result, (err) => {
  this._finishNavigating();
- return err;
+ throw err;
  });
-
- return result;
  });
  }