liblbx: Properly report errors in lbx_(img_)fopen.

The lbx_fopen and lbx_img_fopen functions were not properly pushing an
error code when they failed, leading to extremely helpful messages of
the form:

  lbximg: failed to open image: Success.

It further turns out that pushing negative errors (i.e., -errno) was
documented but never actually tested or used, so fix that up, too.
It is likely that many other sites are lacking appropriate calls to
lbx_error_raise, which can be added later.