issues #60

I think for this version we should not allow additional keys in the error_domain. Any other additions can be put in the extension_domain
'It's RECOMMEND that the keys directly under empirical_error and algorithmic_error use a full URI for their "$ref": value. Resolving the URI SHOULD give a JSON Schema or textual definition of the field. At this time it is RECOMMENDED that no other keys be used in this domain"