Fix spawning of the error reporting goroutine

It has to be spawned *before* we run the main loop, because otherwise if
we had too many errors this would have blocked them.  (Where too many is
`2 * config.concurrency`.  After *all* fetching goroutines would be
blocked and could never resume because the `errors` channel is buffered
as well.  That shouldn't happen now.)

go/stars/stars.go

	sem := make(chan bool, config.concurrency)
	var wg sync.WaitGroup
	hadErrors := false
	errors := make(chan error, config.concurrency)
	go func() {
		for err := range errors {
			hadErrors = true
			fmt.Fprintln(os.Stderr, "Error:", err)
		}
	}()
	wg.Add(len(stars))
	for _, info := range stars {
		info := info
		}()
	}
	hadErrors := false
	go func() {
		for err := range errors {
			hadErrors = true
			fmt.Fprintln(os.Stderr, "Error:", err)
		}
	}()
	wg.Wait()
	close(errors)