package_deb.bbclass: put the stderr into log.do_xxx-attemptonly.xxx
Currently the stderr msgs go into log.do_rootfs and can cause a fatal failure
in "log_check rootfs".
However, the err msg could be spurious since we *only attempt* to install the
the packages, which may not exist actually. E.g., without this patch, we can
get a fatal ERROR due to
E: Couldn't find package task-core-tools-dev.
