func ExampleThrottler_errors() error {
	var urls = []string{
		"http://www.golang.org/",
		"http://www.google.com/",
		"http://www.somestupidname.com/",
	}
	t := New(2, len(urls))
	for _, url := range urls {
		go func(url string) {
			defer t.Done(nil)
			http.Get(url)
		}(url)
		t.Throttle()
	}
	if t.Err() != nil {
		for i, err := range t.Errs() {
			fmt.Printf("error #%d: %s", i, err)
		}
		return t.Err()
	}
	return nil
}
This example fetches several URLs concurrently,
using a Throttler to block until all the fetches are complete
and checks the errors returned.
Compare to http://golang.org/pkg/sync/#example_WaitGroup
package throttler
import "fmt"
type httpPkg struct{}
func (httpPkg) Get(url string) error { return nil }
var http httpPkg
func ExampleThrottler() {
	var urls = []string{
		"http://www.golang.org/",
		"http://www.google.com/",
		"http://www.somestupidname.com/",
	}
	t := New(2, len(urls))
	for _, url := range urls {
		go func(url string) {
			err := http.Get(url)
			t.Done(err)
		}(url)
		errorCount := t.Throttle()
		if errorCount > 0 {
			break
		}
	}
}