Related agreements: Improve error management when the user attempts to link an agreement to itself

An agreement can have related agreements. The linking process currently allows the user to successfully select an agreement for linking to itself, but when the user attempts to save the Agreement, this is blocked and in the linked agreement card an error message is displayed.

It would be preferable if the user was immediately shown an error on selecting the agreement

What currently happens

1. In the agreements app, view a saved agreement.
2. Select to "Edit" the agreement.
3. In the "Related agreements" accordion select "Link agreement" to open the plugin.
4. In the list of agreements select the agreement which is being currently edited (the plugin closes).
5. Notice how the selection appears to have been successful because the agreement card is displayed in the Related agreements edit pane.
6. Save the agreement. An error is displayed as shown below:

What should happen

1. In the agreements app, view a saved agreement.
2. Select to "Edit" the agreement.
3. In the "Related agreements" accordion select "Link agreement" to open the plugin.
4. In the list of agreements select the agreement which is being currently edited (the plugin closes).
5. The selected agreement is not displayed. Instead, an error is displayed in the Agreement card as show in the below mockup.