os/exec: include stderr in ExitError

We should include Stderr in ExitError, so you can write code like:

``` go
stdout, err := exec.Command("foo", "fail").Output()
if err != nil {
     log.Fatalf("foo command failed: %v: %s", err, err.(*exec.ExitError).Stderr)
}
```

Sketch without tests at:
https://go-review.googlesource.com/11415

/cc @rsc
