The following server side validation code will create json error response that is probably designed to work great with unobtrusive asp.net validation but it may not be the best choice for a custom SPA implemented in AngularJS. Notice, that it would be hard to iterate over all properties that contain validation errors.
Fortunately, it is easy to design your own json format by just using CreateResponse call instead of CreateErrorResponse. The simplest, naive example would be to return flat list of validation errors: