Improve error message when repeat specified on entities sheet doesn't exist #784
Description
When declaring an Entity, if you reference a repeat name that doesn't exist in the repeat column, you get an error like:
On the 'survey' sheet, the 'save_to' value 'xyz' is invalid. The entity property populated with 'save_to' must be inside of the entity repeat.
This is technically correct but doesn't help find the issue. It would be more helpful to get a message like
On the 'entities' sheet, the 'repeat' value is set to '${foo}' but there is no repeat by that name.