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/state_linux.go

@@ -64,11 +64,7 @@ func runPoststopHooks(c *Container) error {
 	}
 	s.Status = specs.StateStopped
 
-	if err := hooks[configs.Poststop].RunHooks(s); err != nil {
-		return err
-	}
-
-	return nil
+	return hooks.Run(configs.Poststop, s)
 }
 
 // stoppedState represents a container is a stopped/destroyed state.