Fix Tag/Entity type validation error in Story intermediate file

[ddhar28]

The tags field in a story intermediate file accepts an array of objects of either tag or entity type. If one or more mandatory fields from both type is not present in the object, Validator gives an error for both the types, which in turn might confuse the user.

Acceptance criteria

• [ ] Provide a meaningful error message to customer to help him fix the story intermediate file accordingly

[ddhar28]

The code currently works for tags and also takes extra keys like external-id, type and slug. So it will work for both tags and entities. Since entities are internal and we expect publishers to use it only in specific cases, more refined errors regarding entities need not be added and this can be left as it is for now. We can come back to this later if we require more entity specific error messages in future.