Implement HasDefaultErrorConsumer

Followup from #5569, and allows you to introspect if the resulting observer has missing error consumption and subsequently supplies a default (throwing) one.

src/main/java/io/reactivex/internal/observers/LambdaObserver.java

@@ -20,9 +20,12 @@
 import io.reactivex.exceptions.*;
 import io.reactivex.functions.*;
 import io.reactivex.internal.disposables.DisposableHelper;
+import io.reactivex.internal.functions.Functions;
+import io.reactivex.observers.HasDefaultErrorConsumer;
 import io.reactivex.plugins.RxJavaPlugins;
 
-public final class LambdaObserver<T> extends AtomicReference<Disposable> implements Observer<T>, Disposable {
+public final class LambdaObserver<T> extends AtomicReference<Disposable>
+        implements Observer<T>, Disposable, HasDefaultErrorConsumer {
 
     private static final long serialVersionUID = -7251123623727029452L;
     final Consumer<? super T> onNext;
@@ -101,4 +104,9 @@ public void dispose() {
     public boolean isDisposed() {
         return get() == DisposableHelper.DISPOSED;
     }
+
+    @Override
+    public boolean hasMissingErrorConsumer() {
+        return onError == Functions.ON_ERROR_MISSING;
+    }
 }

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

@@ -0,0 +1,18 @@
+package io.reactivex.observers;
+
+import io.reactivex.annotations.Experimental;
+
+/**
+ * An interface that indicates that the implementing type has default implementations for error consumption.
+ */
+@Experimental
+public interface HasDefaultErrorConsumer {
+
+    /**
+     * @return {@code true} if the implementation is missing an error consumer and thus using a throwing default
+     * implementation. Returns {@code false} if a concrete error consumer implementation was supplied.
+     */
+    @Experimental
+    boolean hasMissingErrorConsumer();
+
+}