Code simplification.

* tools/dist/release.py:
  (post_candidates, create_tag, clean_dist, move_to_dist):
    Simplify use of 'subprocess'.  This gains error checking that was missing.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1485548 13f79535-47bb-0310-9956-ffa450edef68