Make toJava(scalaFuture).toCF.obtrude* unsupported

The status quo was to allow the wrapper and the wrapped to diverge;
the as shown by:

```
    @test
    public void testToJavaToCompletableFutureJavaObtrudeCalledAfterScalaComplete() throws Exception {
        final Promise<String> p = promise();
        Future<String> sf = p.future();
        final CompletionStage<String> cs = toJava(sf);
        CompletableFuture<String> cf = cs.toCompletableFuture();
        assertEquals("notyet", cf.getNow("notyet"));
        p.success("scaladone");
        assertEquals("scaladone", cf.get());
        cf.obtrudeValue("javadone");
        assertEquals("javadone", cf.get());
        // obtrude does not mutate the underlying Scala future
        assertEquals("scaladone", Await.result(sf, Duration.Inf()));
    }
```

This commit throws an UOE, preferring to fail fast.

Author: retronym

src/main/scala/scala/concurrent/java8/FutureConvertersImpl.scala

@@ -72,9 +72,11 @@ object FuturesConvertersImpl {
      * WARNING: completing the result of this method will not complete the underlying
      *          Scala Future or Promise (ie, the one that that was passed to `toJava`.)
      */
-    override def toCompletableFuture(): CompletableFuture[T] = {
-      this
-    }
+    override def toCompletableFuture(): CompletableFuture[T] = this
+
+    override def obtrudeValue(value: T): Unit = throw new UnsupportedOperationException("obtrudeValue may not be used on the result of toJava(scalaFuture)")
+
+    override def obtrudeException(ex: Throwable): Unit = throw new UnsupportedOperationException("obtrudeException may not be used on the result of toJava(scalaFuture)")
 
     override def get(): T = scala.concurrent.blocking(super.get())
 

src/test/java/scala/compat/java8/FutureConvertersTest.java

@@ -378,4 +378,24 @@ public void testToJavaToCompletableFutureJavaCompleteCalledBeforeScalaComplete()
         p.success("scaladone");
         assertEquals("javadone", cf.get());
     }
+
+    @Test
+    public void testToJavaToCompletableFutureJavaObtrudeCalledBeforeScalaComplete() throws ExecutionException, InterruptedException {
+        final Promise<String> p = promise();
+        Future<String> sf = p.future();
+        final CompletionStage<String> cs = toJava(sf);
+        CompletableFuture<String> cf = cs.toCompletableFuture();
+        try {
+            cf.obtrudeValue("");
+            fail();
+        } catch (UnsupportedOperationException iae) {
+            // okay
+        }
+        try {
+            cf.obtrudeException(new Exception());
+            fail();
+        } catch (UnsupportedOperationException iae) {
+            // okay
+        }
+    }
 }