Return proper error while failing to update a User's profile

[isabelcosta]

Description

As reported in issue #83, when a User tries to update its username to an already existing one, the server responds with 500 - Internal Server Error. This happens because the Database model is aware that the username should be unique, hence it fails at saving the User new data.

This will require extra checks in DAO function that updates the User profile.

Acceptance Criteria

Update

• Return proper error message when a user tries to update the username to an already existing one

Definition of Done

• All of the required items are completed.
• PR should be sent to the gsoc18-code branch.
• Approval by 1 mentor.

Estimation

1 hours