Update assertError when expected value is nil (quii#299)

maps.md

@@ -368,9 +368,22 @@ func TestAdd(t *testing.T) {
         assertDefinition(t, dictionary, word, definition)
     })
 }
+...
+func assertError(t *testing.T, got, want error) {
+	t.Helper()
+	if got != want {
+		t.Errorf("got %q want %q", got, want)
+	}
+	if got == nil {
+		if want == nil {
+			return
+		}
+		t.Fatal("expected to get an error.")
+	}
+}
 ```
 
-For this test, we modified `Add` to return an error, which we are validating against a new error variable, `ErrWordExists`. We also modified the previous test to check for a `nil` error.
+For this test, we modified `Add` to return an error, which we are validating against a new error variable, `ErrWordExists`. We also modified the previous test to check for a `nil` error, as well as the `assertError` function.
 
 ## Try to run test