Set-Email operations do not check if email exists

Problem/Motivation

The InitSetEmailResource and SetEmailResource REST Resources do not check if the email to update to already exists on another user account.

This leads to the initialization of the operation to work without problems, but actually setting the email leads to SQL errors.

Proposed resolution

Check for existing emails in the InitSetEmailResource.
Validate the user entity and check for email constraint violations in SetEmailResource.