libct: better errors for hooks

When a hook has failed, the error message looks like this:

> error running hook: error running hook #1: exit status 1, stdout: ...

The two problems here are:
1. it is impossible to know what kind of hook it was;
2. "error running hook" stuttering;

Change that to

> error running createContainer hook #1: exit status 1, stdout: ...

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/rootfs_linux.go

@@ -143,7 +143,7 @@ func prepareRootfs(pipe *os.File, iConfig *initConfig, mountFds mountFds) (err e
 	s := iConfig.SpecState
 	s.Pid = unix.Getpid()
 	s.Status = specs.StateCreating
-	if err := iConfig.Config.Hooks[configs.CreateContainer].RunHooks(s); err != nil {
+	if err := iConfig.Config.Hooks.Run(configs.CreateContainer, s); err != nil {
 		return err
 	}