2.x: explain null observer/subscriber error reports in detail from RxJavaPlugins

src/main/java/io/reactivex/Completable.java

@@ -2168,6 +2168,8 @@ public final void subscribe(CompletableObserver observer) {
 
             observer = RxJavaPlugins.onSubscribe(this, observer);
 
+            ObjectHelper.requireNonNull(observer, "The RxJavaPlugins.onSubscribe hook returned a null CompletableObserver. Please check the handler provided to RxJavaPlugins.setOnCompletableSubscribe for invalid null returns. Further reading: https://github.com/ReactiveX/RxJava/wiki/Plugins");
+
             subscribeActual(observer);
         } catch (NullPointerException ex) { // NOPMD
             throw ex;

src/main/java/io/reactivex/Flowable.java

@@ -14473,7 +14473,7 @@ public final void subscribe(FlowableSubscriber<? super T> s) {
         try {
             Subscriber<? super T> z = RxJavaPlugins.onSubscribe(this, s);
 
-            ObjectHelper.requireNonNull(z, "Plugin returned null Subscriber");
+            ObjectHelper.requireNonNull(z, "The RxJavaPlugins.onSubscribe hook returned a null FlowableSubscriber. Please check the handler provided to RxJavaPlugins.setOnFlowableSubscribe for invalid null returns. Further reading: https://github.com/ReactiveX/RxJava/wiki/Plugins");
 
             subscribeActual(z);
         } catch (NullPointerException e) { // NOPMD

src/main/java/io/reactivex/Maybe.java

@@ -4150,7 +4150,7 @@ public final void subscribe(MaybeObserver<? super T> observer) {
 
         observer = RxJavaPlugins.onSubscribe(this, observer);
 
-        ObjectHelper.requireNonNull(observer, "observer returned by the RxJavaPlugins hook is null");
+        ObjectHelper.requireNonNull(observer, "The RxJavaPlugins.onSubscribe hook returned a null MaybeObserver. Please check the handler provided to RxJavaPlugins.setOnMaybeSubscribe for invalid null returns. Further reading: https://github.com/ReactiveX/RxJava/wiki/Plugins");
 
         try {
             subscribeActual(observer);

src/main/java/io/reactivex/Observable.java

@@ -12079,7 +12079,7 @@ public final void subscribe(Observer<? super T> observer) {
         try {
             observer = RxJavaPlugins.onSubscribe(this, observer);
 
-            ObjectHelper.requireNonNull(observer, "Plugin returned null Observer");
+            ObjectHelper.requireNonNull(observer, "The RxJavaPlugins.onSubscribe hook returned a null Observer. Please change the handler provided to RxJavaPlugins.setOnObservableSubscribe for invalid null returns. Further reading: https://github.com/ReactiveX/RxJava/wiki/Plugins");
 
             subscribeActual(observer);
         } catch (NullPointerException e) { // NOPMD

src/main/java/io/reactivex/Single.java

@@ -3427,7 +3427,7 @@ public final void subscribe(SingleObserver<? super T> observer) {
 
         observer = RxJavaPlugins.onSubscribe(this, observer);
 
-        ObjectHelper.requireNonNull(observer, "subscriber returned by the RxJavaPlugins hook is null");
+        ObjectHelper.requireNonNull(observer, "The RxJavaPlugins.onSubscribe hook returned a null SingleObserver. Please check the handler provided to RxJavaPlugins.setOnSingleSubscribe for invalid null returns. Further reading: https://github.com/ReactiveX/RxJava/wiki/Plugins");
 
         try {
             subscribeActual(observer);

src/test/java/io/reactivex/flowable/FlowableSubscriberTest.java

@@ -815,7 +815,7 @@ public Subscriber apply(Flowable a, Subscriber b) throws Exception {
                 Flowable.just(1).test();
                 fail("Should have thrown");
             } catch (NullPointerException ex) {
-                assertEquals("Plugin returned null Subscriber", ex.getMessage());
+                assertEquals("The RxJavaPlugins.onSubscribe hook returned a null FlowableSubscriber. Please check the handler provided to RxJavaPlugins.setOnFlowableSubscribe for invalid null returns. Further reading: https://github.com/ReactiveX/RxJava/wiki/Plugins", ex.getMessage());
             }
         } finally {
             RxJavaPlugins.reset();

src/test/java/io/reactivex/observable/ObservableSubscriberTest.java

@@ -205,7 +205,7 @@ public Observer apply(Observable a, Observer b) throws Exception {
                 Observable.just(1).test();
                 fail("Should have thrown");
             } catch (NullPointerException ex) {
-                assertEquals("Plugin returned null Observer", ex.getMessage());
+                assertEquals("The RxJavaPlugins.onSubscribe hook returned a null Observer. Please change the handler provided to RxJavaPlugins.setOnObservableSubscribe for invalid null returns. Further reading: https://github.com/ReactiveX/RxJava/wiki/Plugins", ex.getMessage());
             }
         } finally {
             RxJavaPlugins.reset();