Change signature of CreateChangeset to return err last
Created by: mrnugget
This is a follow-up to #7328 and changes the signature of the
CreateChangeset
method according to @tsenart's suggestions.
I'm going to leave the CreateChangeset
method as it is right now,
instead of turning it into an UpsertChangeset
method, since we don't
have BitbucketServer support yet and I'm not sure the interface would be
better.