Descriptive error message when Create User fails

[Yash Bathia]

Currently, when we try to create a new user with an existing User Name, the process fails and displays an error message "Error: Unable to create user.". This error message is insufficient to inform the user about what exactly went wrong ( same User Name). Since we allow users with same email address to exist but not the same User Name, the error message should be specific to the problem "Error: User name xyz is already used" so that the user knows that its the same User Name thats causing the error and he needs to change the User Name and not worry about changing other fields in the new user creation form.

Steps to reproduce the bug:
1. Create new user with User Name "xyz" (Success)
2. Create another user with User Name "xyz1" (Error: Unable to create user")

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/143252

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/143252
Committed: https://git.openstack.org/cgit/openstack/horizon/commit/?id=0dc91bed5ce176330ee20f63be68a4076c534fa5
Submitter: Jenkins
Branch: master

commit 0dc91bed5ce176330ee20f63be68a4076c534fa5
Author: Yash Bathia <email address hidden>
Date: Fri Dec 19 19:39:12 2014 -0800

    Creating a new user with an existing user name

    Change error message when we create a new user
    with an existing user name.

    Change-Id: I795c04672a3c625be074c8af78eb169436ac3895
    Closes-Bug: #1404432