lib/oe/package_manager: turn postinst failure warnings into bitbake failures

lib/oe/package_manager: turn postinst failure warnings into bitbake failures

Sumo release provides a transition period so that deferrals to first boot
via 'exit 1' can be converted to pkg_postinst_ontarget(). For the next release
however, postinst script failures should be treated as such.

[YOCTO #12607]

Signed-off-by: Alexander Kanavin <alexander.kanavin@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>