fix(@angular-devkit/architect): default to failure if no builder result is provided

Currently, if an architect builder does not provide any results, the CLI will crash
trying to access an error message property on the result. Instead architect will now
provide a default failure result `{ success: false }` in the event that the builder
exits prior to generating a result. Thrown errors continue to be propagated as before.

Author: clydin

packages/angular_devkit/architect/src/create-builder.ts

@@ -8,7 +8,7 @@
 
 import { json, logging } from '@angular-devkit/core';
 import { Observable, Subscription, from, isObservable, of, throwError } from 'rxjs';
-import { mergeMap, tap } from 'rxjs/operators';
+import { defaultIfEmpty, mergeMap, tap } from 'rxjs/operators';
 import {
   BuilderContext,
   BuilderHandlerFn,
@@ -218,6 +218,7 @@ export function createBuilder<OptT = json.JsonObject, OutT extends BuilderOutput
         subscriptions.push(
           result
             .pipe(
+              defaultIfEmpty({ success: false } as unknown),
               tap(() => {
                 progress({ state: BuilderProgressState.Running, current: total }, context);
                 progress({ state: BuilderProgressState.Stopped }, context);