2.x: fix TestSubscriber/Observer message texts (#4921)

Author: akarnokd

src/main/java/io/reactivex/observers/TestObserver.java

@@ -149,7 +149,7 @@ public void onNext(T t) {
         values.add(t);
 
         if (t == null) {
-            errors.add(new NullPointerException("onNext received a null Subscription"));
+            errors.add(new NullPointerException("onNext received a null value"));
         }
 
         actual.onNext(t);

src/main/java/io/reactivex/subscribers/TestSubscriber.java

@@ -207,7 +207,7 @@ public void onNext(T t) {
         values.add(t);
 
         if (t == null) {
-            errors.add(new NullPointerException("onNext received a null Subscription"));
+            errors.add(new NullPointerException("onNext received a null value"));
         }
 
         actual.onNext(t);
@@ -226,7 +226,7 @@ public void onError(Throwable t) {
             errors.add(t);
 
             if (t == null) {
-                errors.add(new IllegalStateException("onError received a null Subscription"));
+                errors.add(new IllegalStateException("onError received a null Throwable"));
             }
 
             actual.onError(t);