[FLINK-20336] [core] Do not silently pass UNRECOGNIZED state mutations

This closes #235.
diff --git a/statefun-flink/statefun-flink-core/src/main/java/org/apache/flink/statefun/flink/core/reqreply/PersistedRemoteFunctionValues.java b/statefun-flink/statefun-flink-core/src/main/java/org/apache/flink/statefun/flink/core/reqreply/PersistedRemoteFunctionValues.java
index 150949a..e5c4b76 100644
--- a/statefun-flink/statefun-flink-core/src/main/java/org/apache/flink/statefun/flink/core/reqreply/PersistedRemoteFunctionValues.java
+++ b/statefun-flink/statefun-flink-core/src/main/java/org/apache/flink/statefun/flink/core/reqreply/PersistedRemoteFunctionValues.java
@@ -83,9 +83,8 @@
             break;
           }
         case UNRECOGNIZED:
-          {
-            break;
-          }
+          throw new IllegalStateException(
+              "Received an UNRECOGNIZED PersistedValueMutation type. This may be caused by a mismatch or incompatibility with the remote function SDK version and the Stateful Functions version.");
         default:
           throw new IllegalStateException("Unexpected value: " + mutate.getMutationType());
       }