Display validation messages

[mietcls]

There should be validation on the add member and add handles. It will be triggered if it is a non-unique name. @ismakutl has hardcoded it, ask for more.

[Dakad]

On backend side, I do some validations on CREATE, UPDATE operations on entities/handles/services.
If the input data is invalid, I send back an error with status 422 (Unprocessable Entity) with :
{
name : 'validation.unique'
}

[mrvdries]

I currently do not have time to finish this feature. I'll have to postpone it to september.