"could not find binary package" error could be more helpful
The error that appears when a binary package isn't found could be more helpful. Currently, the error message implies that this is some sort of impossible condition given that everything is working, which isn't true. Merely having build_pkgs_on_install set to False will raise this error when an unbuilt package is encountered. I would propose we amend this error to suggest enabling build_pkgs_on_install if it is set to False when this condition is reached.
Alternatively, we could always build packages where no binary package exists in the repo. I think this would make sense too, given that the hint we provide to the user when they select an option sounds more like saying no just will give you faster installation, not prevent you from running the install command at all in certain conditions.
What are your thoughts?