Add the changelog entry for #1578

changelog.d/1578-space-missions-failed-entry.md

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+- Space Missions no longer locks the workspace when its mission index fails to
+  load. A 429, timeout or bad payload on `/api/launches` now rolls the entry
+  back to the previous layers and camera view instead of leaving an empty,
+  isolating mode, and the roster reads `30-DAY MISSION INDEX UNAVAILABLE`
+  rather than loading forever (German, @gekh, #1578).