adding errors descriptor file + respond_with errors functionality

@fosrias @jlo188 @sheavalentine-mdsol This adds support for errors in crichton-demo-service. Something that we should have done long time ago. 

Also, bumped up version of crichton, now points to 0.0.3.
Also, known issue that rdlint doesn't pass validation for errors API doc, because of missing `routes` section. Something we haven't solved yet.

@fosrias Updated. Please take look and merge.