_selinux.setexec: improve failure message (525726)

This converts "OSError: [Errno 22] Invalid argument" into a more
meaningful error message that is designed to guide users in the
right direction. Also, add an import that was missing for the "sys"
module, since it is referenced in the error paths of many functions
in this module.

X-Gentoo-Bug: 525726
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=525726
Acked-by: Brian Dolbec <dolsen@gentoo.org>
Acked-by: Zac Medico <zmedico@gentoo.org>