Improve response and give better error while adding user in mscolab.

[Tanish Grover]

The error currently says invalid email or taken. There can be cases where the username is taken and not the email.

[Reimar Bauer]

we have to verify that we can only have uniq usernames, as all the right managment is based on that

[Tanish Grover]

The check for having unique usernames already exists in the user registration end point.

[Reimar Bauer]

improve server response while adding user in mscolab, fixes #537

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