We do not support out-of-tree building of rpm-pkg / deb-pkg.  If O=
is given, the build should be terminated, but the "false" command is
not effective since it is not the last command in the cmd_src_tar.
Then, rpm-pkg / deb-pkg tries to continue building, and fails for a
different reason.

Set -e option so that the "false" terminates the building immediately.

I also put the error messages to stderr, and made it stand out more.

For example, "make O=foo rpm-pkg" will fail as follows:

  /bin/bash ../scripts/package/mkspec >./kernel.spec
    TAR     kernel-4.15.0_rc2+.tar.gz

    ERROR:
    Building source tarball is not possible outside the
    kernel source tree. Don't set KBUILD_OUTPUT, or use the
    binrpm-pkg or bindeb-pkg target instead.

  ../scripts/package/Makefile:53: recipe for target 'rpm-pkg' failed
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Jim Davis <jim.epost@gmail.com>