Group all unknown reference errors
Show all error messages

...instead of only the last one in the list. Adjusted CSS to prevent a huge error message from flooding the screen and making it impossible to access the close button (x) on the error message.
Fix the group-move bug

References to group's children were not being updated. A new updatePaths() method was added because the updatePath() method clobbered the LogicManager's internal state while updating the first path:

setMugPropertyValue
    -> property-changed event
    -> LogicManager.updateReferences(mug, propertyPath)
    -> LogicManager.all items for mug are cleared,
      and new items added, but paths are now invalid so ref === ""
Fix bad english