Stopping on the First Error

Three fetches start at the same time. The first returns its page after 100 ms. The second learns after 200 ms that its page does not exist. The third is slow and needs a full second. A caller that needs all three pages has failed at the 200 ms mark, yet the version that waits for every goroutine returns only after 1,000 ms, five times as long as necessary.

In this lesson's fetch, any URL containing /slow takes one second, and a URL ending in /missing fails after 200 ms