Add rx error learning test

domain/src/test/java/org/mercury_im/messenger/learning_tests/rx/OnErrorTest.java

@@ -0,0 +1,38 @@
+package org.mercury_im.messenger.learning_tests.rx;
+
+import org.junit.Test;
+
+import io.reactivex.Observable;
+import io.reactivex.Single;
+
+public class OnErrorTest {
+
+    @Test
+    public void observableErrorTest() throws InterruptedException {
+        Observable<String> errorObservable = Observable.error(new AssertionError());
+        Observable<String> fallbackObservable = Observable.just("String");
+
+        errorObservable
+                .doOnError(e -> System.out.println("Caught here."))
+                .onErrorResumeNext(fallbackObservable)
+                .flatMap(s -> Observable.just(s)
+                                .doOnError(e -> System.out.println("Or here?")))
+                .doOnError(e -> System.out.println("Or even here?"))
+                .subscribe(s -> System.out.println("Result: " + s),
+                        e -> System.out.println("Finally here."));
+        Thread.sleep(100);
+    }
+
+    @Test
+    public void singleErrorTest() throws InterruptedException {
+        Single<String> errorSingle = Single.error(new AssertionError());
+        Single<String> successSingle = Single.just("Item");
+
+        successSingle
+                .map(s -> "Map: " + s)
+                .onErrorResumeNext(Single.just("cancel"))
+                .subscribe(s -> System.out.println(s),
+                        e -> System.out.println("Error"));
+        Thread.sleep(100);
+    }
+}