Version of Append that does not flatten

I realize that `Append()` was fixed at some point to flatten, but I believe it's valuable to have a version of it that does not flatten MultiErrors. Use case: execute a bunch of tasks in parallel, wait for them to finish, collect the errors from the failed tasks and return them to the caller as a single MultiError. I would like to be able to preserve any MutliError generated by each task rather than ending up with a flat list of errors that lost their context, the demarcation points between tasks if you will.