Improve error message when adding the wrong value to a property that accepts several different kinds of linked/embedded types

[ehennestad]

Example: Adding a string to the memberOf property of the Affiliation

openminds.core.Affiliation('memberOf', 'test')

Fails with error saying value should be of type openminds.internal.mixedtype.affiliation.MemberOf.

Error using [openminds.core.actors.Affiliation
 openminds.core.Affiliation('memberOf', 'test')
                                        ^^^^^^
Invalid value for 'memberOf' argument. Value must be openminds.internal.mixedtype.affiliation.MemberOf or be convertible to openminds.internal.mixedtype.affiliation.MemberOf.

This is an internal container. The error message should list the actual allowed types, i.e.,

• openminds.core.actors.Consortium
• openminds.core.actors.Organization