feat(ObjectUnsubscribedError): add ObjectUnsubscribed error class

this class was added to support throwing errors when members of Subjects were accessed in invalid ways after the Subject has been unsubscribed

related #859
related #758

src/Rx.KitchenSink.ts

@@ -137,6 +137,7 @@ import {BehaviorSubject} from './subject/BehaviorSubject';
 import {ConnectableObservable} from './observable/ConnectableObservable';
 import {Notification} from './Notification';
 import {EmptyError} from './util/EmptyError';
+import {ObjectUnsubscribedError} from './util/ObjectUnsubscribedError';
 import {ArgumentOutOfRangeError} from './util/ArgumentOutOfRangeError';
 import {nextTick} from './scheduler/nextTick';
 import {immediate} from './scheduler/immediate';
@@ -167,6 +168,7 @@ export {
     Notification,
     EmptyError,
     ArgumentOutOfRangeError,
+    ObjectUnsubscribedError,
     TestScheduler,
     VirtualTimeScheduler,
     TimeInterval

src/Rx.ts

@@ -117,7 +117,7 @@ import {ConnectableObservable} from './observable/ConnectableObservable';
 import {Notification} from './Notification';
 import {EmptyError} from './util/EmptyError';
 import {ArgumentOutOfRangeError} from './util/ArgumentOutOfRangeError';
-import {nextTick} from './scheduler/nextTick';
+import {ObjectUnsubscribedError} from './util/ObjectUnsubscribedError';
 import {immediate} from './scheduler/immediate';
 import {NextTickScheduler} from './scheduler/NextTickScheduler';
 import {ImmediateScheduler} from './scheduler/ImmediateScheduler';
@@ -142,5 +142,6 @@ export {
     ConnectableObservable,
     Notification,
     EmptyError,
-    ArgumentOutOfRangeError
+    ArgumentOutOfRangeError,
+    ObjectUnsubscribedError
 };

src/util/ObjectUnsubscribedError.ts

@@ -0,0 +1,10 @@
+/**
+ * an error thrown when an action is invalid because the object
+ * has been unsubscribed
+ */
+export class ObjectUnsubscribedError extends Error {
+  constructor() {
+    super('object unsubscribed');
+    this.name = 'ObjectUnsubscribedError';
+  }
+}