The session `WithTransaction` currently tries to cast the error returned by the user supplied function to see if it is a `CommandError`. I believe it should instead use `errors.As()` to see if the returned error unwraps to a command error so that if a developer wraps an error(ie so you can return a prettier error to users) then the command error will still be detected by the mongodb driver and retry logic still applied.

This block specifically the first line with `err.(CommandError)` should be changed to use `errors.As` instead which will check if the error is a CommandError directly or if it unwraps to one and contains a CommandError.

https://github.com/mongodb/mongo-go-driver/blob/master/mongo/session.go#L196-L200

Example use case: https://play.golang.org/p/pOkZ7a9anzj

https://golang.org/pkg/errors/#As

I believe this is the cause of WriteConflict errors when we moved transactions to production.