Issue Description: 

Validation within RHOSE's public API endpoints has the potential to return multiple errors. For this reason we worked on support for a ErrorsResponse. However the methods themselves still have the potential to return a single ErrorResponse object. This makes defining rhose-api.yaml compliant with AppServices impossible as a single response type must be defined.

Acceptance Criteria:

Change all public API methods to return ErrorsResponse. In the event that a single error needs to be returned the ErrorsResponse will contain a single item.

Out of Scope: 

UI re-work.

Additional Information: 

See here for the discussion.