Never write errors to stdout from ipkg-make-index.sh

Never write errors to stdout from ipkg-make-index.sh

Since the output of ipkg-make-index.sh is unconditionally used to create
the Packages file, sending the errors into this file is unacceptable
(both because they are not shown to users and because they make an
invalid Packages file).

For incorrect invocation, write to stderr.
For errors from which, send to /dev/null with stdout.

Signed-off-by: Kevin Locke <klocke@digitalenginesoftware.com>
SVN-Revision: 25677