Add DCStatus data type for the data collectors

Also adds the DCStatusCode, part of DCStatus, and the addStatus
utility function for adding the "status" field to an already existing
JSValue.

The design document is updated to have the status codes sorted by increasing
seriousness.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>