2.x: Add explanation text to Undeliverable & OnErrorNotImplemented exs (#6171)

* 2.x: Add explanation text to Undeliverable & OnErrorNotImplemented exs

* Link to the wiki instead of the docs directory due to broken [[]] links

* Reword OnErrorNotImplemented

Author: akarnokd

src/main/java/io/reactivex/exceptions/OnErrorNotImplementedException.java

@@ -48,6 +48,6 @@ public OnErrorNotImplementedException(String message, @NonNull Throwable e) {
      *          the {@code Throwable} to signal; if null, a NullPointerException is constructed
      */
     public OnErrorNotImplementedException(@NonNull Throwable e) {
-        super(e != null ? e.getMessage() : null, e != null ? e : new NullPointerException());
+        this("The exception was not handled due to missing onError handler in the subscribe() method call. Further reading: https://github.com/ReactiveX/RxJava/wiki/Error-Handling | " + (e != null ? e.getMessage() : ""), e);
     }
 }

src/main/java/io/reactivex/exceptions/UndeliverableException.java

@@ -28,6 +28,6 @@ public final class UndeliverableException extends IllegalStateException {
      * @param cause the cause, not null
      */
     public UndeliverableException(Throwable cause) {
-        super(cause);
+        super("The exception could not be delivered to the consumer because it has already canceled/disposed the flow or the exception has nowhere to go to begin with. Further reading: https://github.com/ReactiveX/RxJava/wiki/What's-different-in-2.0#error-handling | " + cause.getMessage(), cause);
     }
 }

src/test/java/io/reactivex/exceptions/ExceptionsTest.java

@@ -53,7 +53,8 @@ public void accept(Integer t1) {
 
         });
 
-        TestHelper.assertError(errors, 0, RuntimeException.class, "hello");
+        TestHelper.assertError(errors, 0, RuntimeException.class);
+        assertTrue(errors.get(0).toString(), errors.get(0).getMessage().contains("hello"));
         RxJavaPlugins.reset();
     }