Image error subfield pictureId should be generalized, context-sensitive

[JamesBeach]

A nullable field in the error type returned to the client called pictureId is nominally intended to carry the unique ID of an image when an image-related error has been generated. It may be desirable that this should instead be generalized so that it can accompany other kinds of errors, when applicable.

For example, it is possible that a client may try to verify a photo or, in later iterations, train further against a facId that does not exist for its corresponding facType. Though this error doesn't yet have a specific code, it probably should. And, to enable the client to take appropriate action, it may be desirable to embed the facType in the error payload.

It may be called simply id, given that it identifies a problematic parameter in the request. While we could maintain fields with different names that show up sometimes and not others--pictureId, facType, and so on--this would introduce additional complications to the JSON parsing or mappings. Simpler to have it be context-sensitive, its meaning derived from its associated error code.