allow raise after explicit rollback within #transaction(&block)

src/db/begin_transaction.cr

@@ -7,16 +7,7 @@ module DB
       begin
         yield tx
       rescue e
-        # TODO check if `unless tx.closed?` should be added.
-        # If it is not added, when an exception occurs after the
-        # transaction is closed (explicit rollback/commit)
-        # the call to rollback will trigger an exception.
-        # Since the code will be in between `#transaction(&block)`
-        # seems reasonable to raise an error since it might trigger
-        # a design flaw. Transaction might been commited already.
-        # Maybe we should wrap e in another exception that clarifies
-        # this scenario.
-        tx.rollback
+        tx.rollback unless tx.closed?
         raise e unless e.is_a?(DB::Rollback)
       else
         tx.commit unless tx.closed?