Report any definitions that reference non-existing definitions

Confirm that all problems with YAML definitions are reported to Definitions app

• Definitions which reference definitions which do not exist.(For example a template definition references a non-existent dialog definition)

Reported messages should say

• Which class / definition / whatever is deprecated / not found

Please note Magnolia's behaviour in those cases currently as a comment or in a wiki page.

Requirement in YAML backwards compatibility