Use the new gimp_get_pdb_status() to forward the error returned by
gimp_file_load(). Previous code was always returning
GIMP_PDB_EXECUTION_ERROR when the file load was failing, but this was
not granular enough. In particular when the file load is actually
interactively cancelled through Esc or the "Cancel" button, we don't
want to display an error message on screen. Therefore we forward the
actual error raised by the underlining plug-in.