Correct default error message.

Author: android10

domain/src/main/java/com/fernandocejas/android10/sample/domain/exception/DefaultErrorBundle.java

@@ -20,6 +20,8 @@
  */
 public class DefaultErrorBundle implements ErrorBundle {
 
+  private static final String DEFAULT_ERROR_MSG = "Unknown error";
+
   private final Exception exception;
 
   public DefaultErrorBundle(Exception exception) {
@@ -33,10 +35,6 @@ public Exception getException() {
 
   @Override
   public String getErrorMessage() {
-    String message = "";
-    if (this.exception != null) {
-      this.exception.getMessage();
-    }
-    return message;
+    return (exception != null) ? this.exception.getMessage() : DEFAULT_ERROR_MSG;
   }
 }