'Transfer repository ownership' does not validate usernames

When a user tries to transfer a repository to another user, they get a pop up window where they need to enter the recipient. It's not specified whether they should enter a username or an email address, however it will only work for usernames. Some users will enter an email address, click Transfer and keep waiting, because they don't get any error message that email addresses are not allowed.

It would be nice to advice them to enter a username instead of email or display an error if the operation couldn't go through.