Unclear error message when @refVia tag is set on column w/o a foreign key
### Summary

When I set a refVia tag on a column without a foreign key, I got the error message `TypeError: Cannot convert undefined or null to object`, which didn't help much towards solving the problem. 

### Steps to reproduce

https://github.com/tinleym/ouch-my-finger_view-union-error

### Possible Solution

It would be nice to instead have the error message inform the user that foreign keys need to be defined on any refVia column.