Blob fixes (#3599)

* Fix error if movie back image blob was not found
* Don't error out if scene cover get fails
* Don't error out on image get fails
* Add debug logging for fs blobs
* Remove old blob data when no longer referenced

pkg/scene/export_test.go

@@ -178,8 +178,9 @@ var scenarios = []basicTestScenario{
 	},
 	{
 		createFullScene(errImageID),
-		nil,
-		true,
+		createFullJSONScene(""),
+		// failure to get image should not cause an error
+		false,
 	},
 }