f clearer debug msg

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -13194,7 +13194,7 @@ where
 							let mut claimable_payments = channel_manager.claimable_payments.lock().unwrap();
 							match claimable_payments.pending_claiming_payments.entry(payment_hash) {
 								hash_map::Entry::Occupied(_) => {
-									debug_assert!(false, "unreachable");
+									debug_assert!(false, "Entry was added in begin_claiming_payment");
 									return Err(DecodeError::InvalidValue);
 								},
 								hash_map::Entry::Vacant(entry) => {