pip doesn't, but the existing `djangopypi` command we piggy-back on does.

sample output:

    ./manage.py pip_add django==1.5.2 --owner=ofspypi
    Django-1.5.2 added

    ./manage.py pip_add mising_qqxx==1.0.0 django==1.5.3 --owner=ofspypi
    Error: No distributions at all found for mising-qqxx==1.0.0

    ./manage.py pip_add django==1.5.3 mising_qqxx==1.0.0 --owner=ofspypi
    Django-1.5.3 added
    Error: No distributions at all found for mising-qqxx==1.0.0

(bails immediately on error and returns non-zero)