archive-tar: report filter start error only once

archive-tar: report filter start error only once

A missing tar filter is reported by start_command() using error(), but
also by its caller, write_tar_filter_archive(), using die():

   $ git -c tar.invalid.command=foo archive --format=invalid HEAD
   error: cannot run foo: No such file or directory
   fatal: unable to start 'foo' filter: No such file or directory

The second message contains all relevant information and even says that
the failed command was intended to be used as a filter.  Silence the
first one because it's redundant.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Taylor Blau <me@ttaylorr.com>