Warn of bad visualization node groups and members.
Print all visualisation warnings to stderr.
Merge pull request #571 from hjoliver/556.vis-validation-warnings

Warn of bad visualization node groups and members.
Fix clock triggered task

cbc86d6b560183c5a281dc6c3b8ec10691a47e02 broke clock triggered tasks.
(It only worked with clock triggered families but not individual tasks.)
Merge pull request #579 from matthewrmshin/fix-clock-trigger-task

Fix clock triggered task