sysroot: Correct error handling path in previous libglnx port
sysroot: Correct error handling path in previous libglnx port

I happened to have the github page open with my commit after it was
merged, and then noticed a bug.

We still need to throw if we hit a non-`ENOENT` error.

Closes: #315
Approved by: giuseppe

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