Fix unhandled refund command

[rocketpages]

TransferEntity needs to be updated to handle com.redelastic.stockbroker.wireTransfer.impl.transfer.TransferCommand$RefundSuccessful. This command occurs when funds cannot be retrieved from the broker successfully.

java.util.concurrent.CompletionException: com.lightbend.lagom.javadsl.persistence.PersistentEntity$UnhandledCommandException: Unhandled command [com.redelastic.stockbroker.wireTransfer.impl.transfer.TransferCommand$RefundSuccessful] in [com.redelastic.stockbroker.wireTransfer.impl.transfer.TransferEntity] with id [63ce4cec-8b59-408d-bf58-e400c7b07fa3]
	at java.util.concurrent.CompletableFuture.encodeRelay(CompletableFuture.java:326)
	at java.util.concurrent.CompletableFuture.completeRelay(CompletableFuture.java:338)
	at java.util.concurrent.CompletableFuture.uniRelay(CompletableFuture.java:911)
	at java.util.concurrent.CompletableFuture.uniCompose(CompletableFuture.java:953)
	at java.util.concurrent.CompletableFuture$UniCompose.tryFire(CompletableFuture.java:926)
	at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:442)
	at java.lang.Thread.run(Thread.java:748)