fix (groups): send error 400 instead of 500 if user does not exist

[maximecaruchet]

No description provided.

[FabienM]

Should this be a 401?

[maximecaruchet]

This is not an authentication problem: the user is authenticated but he is trying to add people who are not registered in TaT, which is an error from the client because the client should add existing user to groups.
That is why I chose a 400, are you OK with that or do you think another status code would be more appropriate?

[FabienM]

That is why I chose a 400, are you OK with that or do you think another status code would be more appropriate?

400 is fine 👌