Throw meaningful error message when trying to return non-objects from stub used as constructor

[fatso83]

Is your feature request related to a problem? Please describe.
See #1676. User's think that not returning a string or other non-object is an error when newing up objects. We should rather explicitly throw an error message saying this is intended.

Describe the solution you'd like
Throw an error

[stale]

Is this still relevant? If so, what is blocking it? Is there anything you can do to help move it forward?

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.