Improve errors handling in groovy templates

[bugzmanov]

Affects: devops, template writers

Background: currently genesis skip templates with errors, and logs is the only way to understand what has happened. This is not acceptable behaviour if we are planning to use genesis widely(especially in embeded mode)

In order to be able to fix template errors, genesis should not skip invalid templates, but show them being broken and try to help template developer in localizing error. It should not be possible to create instance via invalid template.