Bias, variance, and leakage in plain code — step 6 of 7

Write diagnose(train_err, test_err) that returns one of four labels: "leakage" if test error is below 0.01 (too good to be true), "underfit" if train error is above 0.3, "overfit" if test error exceeds train error by more than 0.2, otherwise "ok". Check them in that order.