[WIP] Return validation errors via the REST API

Enable a structured validation error response (422) that returns each validation error.  Normalize the field value for validation errors so that it corresponds to the real JSON value (rather than the internal value).  Use the same constants in api/types.go and api/errors/errors.go for validation errors.
I prefer explicitly calling out field as part of the CauseReasonType.  Prefix with field when its related to a field and postfix with Value when its the passed value that caused the problem.