fix(aio): ensure DocumentService doesn't crash on bad JSON

aio/src/app/documents/document.service.spec.ts

@@ -105,6 +105,22 @@ describe('DocumentService', () => {
       expect(currentDocument).toEqual(nextDoc);
     });
 
+    it('should not crash the app if the response is not valid JSON', () => {
+      let latestDocument: DocumentContents;
+      const { service, backend, location } = getServices('initial/url');
+      const connections = backend.connectionsArray;
+
+      service.currentDocument.subscribe(doc => latestDocument = doc);
+
+      connections[0].mockRespond(new Response(new ResponseOptions({ body: 'this is invalid JSON' })));
+      expect(latestDocument.title).toEqual('Error fetching document');
+
+      const doc1 = { title: 'doc 1' };
+      location.urlSubject.next('new/url');
+      connections[1].mockRespond(createResponse(doc1));
+      expect(latestDocument).toEqual(doc1);
+    });
+
     it('should not make a request to the server if the doc is in the cache already', () => {
       let latestDocument: DocumentContents;
       let subscription: Subscription;

aio/src/app/documents/document.service.ts

@@ -54,7 +54,8 @@ export class DocumentService {
             return Observable.of({ title: 'Not Found', contents: 'Document not found' });
           }
         } else {
-          throw error;
+          this.logger.error('Error fetching document', error);
+          return Observable.of({ title: 'Error fetching document', contents: 'Sorry we were not able to fetch that document.' });
         }
       })
       .subscribe(subject);