validations return list of errors, not just the first one [#81793600]

Signed-off-by: Nick Wade <nwade@pivotal.io>
BBS calls validate implicitly [#81793600]

Signed-off-by: Nick Wade <nwade@pivotal.io>
factor out Validate methods from all JSON unmarshal helpers [#81793600]

Signed-off-by: Nick Wade <nwade@pivotal.io>