Better test failure message

src/bin/flat/main.rs

@@ -796,7 +796,7 @@ fn test_rect_metric_derivs() {
 			let computed = testee.part_derivs_at(pos);
 			let reference = approx.part_derivs_at(pos);
 			let eq = (0..2).all(|coord| computed[coord].abs_diff_eq(reference[coord], epsilon));
-			assert!(eq, "At {}:\nactual:   {:?}\nexpected: {:?}\n", pos, computed, reference);
+			assert!(eq, "Bad derivative computation at {}:\n  explicit:  {:?}\n  numerical: {:?}\n", pos, computed, reference);
 		}
 	}
 }