insights: nice error message when attempting to live preview empty repos

Created by: chwarwick

Return the error message that the UI previously showed when repos where empty during live preview. Error will only display if all repos are empty.

resolves https://github.com/sourcegraph/sourcegraph/issues/35450 for search insights. Opened https://github.com/sourcegraph/sourcegraph/issues/37047 for language stats.

Test plan