Add relevant exception message for checkArgumentNotNull usage

I ran into this while writing some other code and having the exception message
would have made it easier to debug.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=199292321

java/google/registry/model/domain/GracePeriod.java

@@ -137,7 +137,7 @@ public class GracePeriod extends ImmutableObject {
       DateTime expirationTime,
       String clientId,
       Key<BillingEvent.Recurring> billingEventRecurring) {
-    checkArgumentNotNull(billingEventRecurring);
+    checkArgumentNotNull(billingEventRecurring, "billingEventRecurring cannot be null");
     return createInternal(type, expirationTime, clientId, null, billingEventRecurring);
   }