kbuild: propagate errors from find in scripts/gen_initramfs_list.sh

If the find(1) in scripts/gen_initramfs_list.sh generates any errors, it
will cause gen_initramfs_list.sh to fail (because of "set -e"), however
the errors from find are not printed to the user. This is rather confusing:

~/src/powerpc$ make O=~/build/powerpc-cell32/
make[2]: *** [usr/initramfs_data.cpio.gz] Error 1
make[1]: *** [usr] Error 2
make[1]: *** Waiting for unfinished jobs....
make[1]: *** wait: No child processes.  Stop.
make: *** [_all] Error 2

It is much easier to work out what the problem is if we let the errors
from find hit the console, eg:

~/src/powerpc$ make O=~/build/powerpc-cell32/
find: /home/michael/initramfs-source/home: Permission denied
find: /home/michael/initramfs-source/lost+found: Permission denied
find: /home/michael/initramfs-source/opt: Permission denied
find: /home/michael/initramfs-source/root: Permission denied
make[2]: *** [usr/initramfs_data.cpio.gz] Error 1
make[1]: *** [usr] Error 2
make[1]: *** Waiting for unfinished jobs....
make[1]: *** wait: No child processes.  Stop.
make: *** [_all] Error 2

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>