transaction: add a message to check for unclosed transaction

I've forgotten to close a transaction a few times and while the
message ('assertion failed: self.closed') is clear to me now, it
probably won't be clear to others or to me in the future.

lib/src/transaction.rs

@@ -195,7 +195,7 @@ impl<'r> Transaction<'r> {
 impl<'r> Drop for Transaction<'r> {
     fn drop(&mut self) {
         if !std::thread::panicking() {
-            assert!(self.closed);
+            assert!(self.closed, "Transaction was dropped without being closed.");
         }
     }
 }