alreadyExists error for create

[neilj]

The alreadyExists SetError is defined in Core for /copy. For Mail at least, this might be something the server wishes to return on create in /set too if it forbids identical content (if it just forbids identical message-id, it probably makes more sense to return invalidProperties). It's unclear if this is a curiosity of mail or might be more generally applicable. If we ever do a rev2 of the spec, we should have enough implementation experience (across multiple domains, not just email) to determine and fully define this.