add LoadSnapshotFailed in snapshot protocol, #21842

* treat snapshot load failure in same way as other recovery failures
* if load of snapshot fails the persistent actor will be stopped, since
  we can't assume that a consistent state would be recovered just by
  replaying all events, since events may have been  deleted
* additional recovery docs
* improve log message

akka-docs/rst/scala/code/docs/persistence/PersistenceDocSpec.scala

@@ -59,6 +59,13 @@ object PersistenceDocSpec {
       }
       //#recovery-completed
     }
+
+    trait MyPersistentActor5 extends PersistentActor {
+      //#recovery-no-snap
+      override def recovery = 
+        Recovery(fromSnapshot = SnapshotSelectionCriteria.None)
+      //#recovery-no-snap
+    }
   }
 
   object PersistenceId {