Improve -Z error reporting.

Improve -Z error reporting.

The most likely reason for setfscreatecon to fail is that you don't have permission, and that's reported by the write return EACCES. There isn't really a "bad" context; they're just strings.

Before:

  $ adb shell mkdir -Z x y
  mkdir: bad -Z 'x'

After:

  $ adb shell mkdir -Z x y
  mkdir: -Z 'x' failed: Permission denied

Other than this, the ToT mkdir works fine with SELinux.