lib/sysroot: Add prefixes to syncfs/fsync error messages
lib/sysroot: Add prefixes to syncfs/fsync error messages

And clean up one other bare `glnx_throw_errno()`.

Closes: #1044
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree