Improve error messaging

As a User Manager in Bentley Cloud, I would like to have more informative messages when something goes wrong. This way, I would not need to create new cases with Bentley Support to find out the origin of the error.

For example, trying to add a user I got the error message 'could not create user'. Explaining why (in this case the user already existed in a different organization) would help me find a work around and not spend valuable time investigating.