Importmap - find by key: throw error if key-attribute is not unique

When defining an import mapping the option to find an object (by key) needs an attribute.

If you select an attribute that is not unique, at runtime this import-mapping will sometimes find two objects and will fail.

This error can be prevented design-time. Hence this suggestion:

For the import map interface - find by key: add an error to the error list at design-time, upon saving an import mapping where the developer selects a key-attribute that is not defined as being unique.