A standard way of reporting validation-errors

[simen]

We need a machine-readable way to report validation-errors. You should be able to wrap a block of active-record stuff and automatically catch any validation errors and forward the message to the caller as a json-hash with the fields and error messages for each. This is e.g. needed in checkpoint /api/v1/identities.rb:10 where a validation error will yield an Internal Server Error.