SLING-4157, detect validation models in arbitrary locations

(only below the resource resolver search paths and below a node named "validation")

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1638874 13f79535-47bb-0310-9956-ffa450edef68
SLING-4143 - preserve original HTTP status and message if error handling script fails

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1638875 13f79535-47bb-0310-9956-ffa450edef68