lib/sysroot: Add some g_prefix_error() for ostree_sysroot_cleanup()
lib/sysroot: Add some g_prefix_error() for ostree_sysroot_cleanup()

We saw this fail in a CI run. We've been trying to add strategic error prefixing
as a general rule, and this specific instance may help debug.

Closes: #929
Approved by: pwithnall

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