Related discussion: https://github.com/ansible/galaxy_ng/pull/562#discussion_r520698907

Problems:

• On galaxy_ng/app/api/v3/viewsets/namespace.py there is a `create` method which creates 2 instances (repo and distro) and also validates for IntegrityError expecting error for unique name constraints.
• The validation there uses a try:except introduced by the PR 562 and this error catching are too general and can lead to exception swallowing if integrity occurs related to a different field.
• On galaxy_ng/app/api/v3/serializers/namespace.py there is also a `create` method and a `validate_name` which is also performing some creation and validation.

Proposed solutions:

• Create a common `create_or_raise` method to be used on every instance creation and then centralizing validation.
• Move the creation of the 3 entities to the same place on the Serializer and after the validator is performed *recommended*
• apply ScopedErrorSerializer https://github.com/ansible/galaxy_ng/blob/master/galaxy_ng/app/api/v3/serializers/namespace.py#L17