Currently, any failed request to the source Wiki will result in the target Wikibase entering an error state. This behavior is the case even in non-critical situations such as displaying property labels on the item page.
If displaying federated properties is not possible when loading the entity page due to a failed call to the source Wiki, we should display a partial version of the entity page and an error message explaining the situation to the user: "Statements about this item are temporarily unavailable due to a failed request to the federation source Wiki. Please refresh the page."
We've decided the entity ID and label in the current language should be displayed, in addition to the error message (just an error page, avoids page rendering complexity).
Given My connection to the source Wiki is unavailable
When I load an item page containing statements with federated properties
Then An error page is displayed containing the item ID, label in the current language, and an error message
Note: this needs to be made testable in the test system for product verification to take place