Fix lchown when no user found

Summary: Also add appropriate warnings

Increases Symfony from 89.68 to 89.8
Closes https://github.com/facebook/hhvm/pull/4194

Reviewed By: @JoelMarcey

Differential Revision: D1669950

Signature: t1:1669950:1415808530:a050c547474714b230cccd9cfd3d638160743d0f
Try to catch more information about a problem

Summary: The bug isn't this code, but this is where it crashes.  Add an assert
and hopefully we'll get more info.

Reviewed By: @alexmalyshev

Differential Revision: D1674370