common interface for common attributes

[deweyjose]

Problem Statement

EntitiesResponse and EntityObjectResponse share a common set of attributes, getter/setters for error and extensions.

Testing the result of handleFroidRequest is more complex than it needs to be. use-case - logging errors.

Proposed Solution

Define a common interface that provides accessors for errors and exentions.

Alternatives Considered

Please briefly describe which alternatives, if any, have been considered,
including merits of alternate approaches and tradeoffs being made.

Additional Context

Please provide any other information that may be relevant.