When this test fails, print out why to assist in debugging

tests/testing.rs

@@ -9,7 +9,12 @@ fn mdbook_can_correctly_test_a_passing_book() {
     let temp = DummyBook::new().with_passing_test(true).build().unwrap();
     let mut md = MDBook::load(temp.path()).unwrap();
 
-    assert!(md.test(vec![]).is_ok());
+    let result = md.test(vec![]);
+    assert!(
+        result.is_ok(),
+        "Tests failed with {}",
+        result.err().unwrap()
+    );
 }
 
 #[test]