Namespace to Gateway - Error messaging updates on Portal

[ikethecoder]

There are various error and informative messages that are generated in the Portal that reference "namespace". These should be updated to be "gateway".

The sibling of this ticket is #1067 which is focused on the UI specifically (under nextapp). The majority of messages come from validation rules on the backend.

Scope:

• include both Cypress and backend changes at same time
• all namespace references in ValidationError exceptions and other validation rules and messages

Technical recommendations: Perhaps branch off dev where cypress is passing, make these changes and then merge them in the feature/ns-to-gw branch afterwards.

[rustyjux]

Though a PR has been merged there are still error messages returned by gwa-api which include namespace. E.g. the message when trying to publish a service with a host name which is already in use in another namespace.

Not sure the urgency or effort of making updates to gwa-api to capture those messages.