[improve][broker] Add a message to a NullPointerException created in …

…ManagedLedgerImpl

- a NPE with no description is confusing

Seeing this in logs of org.apache.pulsar.client.api.SubscriptionMessageDispatchThrottlingTest#testMultiLevelDispatch

2022-08-26T11:18:10,532 - ERROR - [broker-topic-workers-OrderedExecutor-3-0:SafeRunnable@38] - Unexpected throwable caught
java.lang.NullPointerException: null

managed-ledger/src/main/java/org/apache/bookkeeper/mledger/impl/ManagedLedgerImpl.java

@@ -3475,7 +3475,7 @@ public PositionImpl getNextValidPositionInternal(final PositionImpl position) {
         while (!isValidPosition(nextPosition)) {
             Long nextLedgerId = ledgers.ceilingKey(nextPosition.getLedgerId() + 1);
             if (nextLedgerId == null) {
-                throw new NullPointerException();
+                throw new NullPointerException("nextLedgerId is null. No valid next position after " + position);
             }
             nextPosition = PositionImpl.get(nextLedgerId, 0);
         }