Add a noDataString function to FeedBuilder

Summary: Feed currently returns nothing is there are no stories, we can present a better view here by allowing a base and customizable set of errors. Fixes T7383

Test Plan:
Test a Project feed with no noDataSting and People with a noDataSting

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7383

Differential Revision: https://secure.phabricator.com/D11897