collections.jinja2: improve exception message for untracked relation

[veprbl]

BEGINRELEASENOTES

• Improved exception message for the case when relation to untracked object is to be persisted.

ENDRELEASENOTES

[tmadlener]

Thanks a lot. I am wondering whether we should also put some more information there, e.g. the name of the relation?

Also, a similar check for single relations doesn't exist. I am no longer sure why we treat them differently, but arguably there should not be a difference, I think.

[veprbl]

That's a good idea. A single relation can be checked too, but that would be a breaking change for the users.

[tmadlener]

Yeah that is true. Let's leave it at the non-breaking improvement for now. I will make a note of figuring out why one-to-one and one-to-many relations are treated differently.