gitcollector should not start when no org is passed

[dpordomingo]

related to src-d/ghsync#54

When source{d} CE is inited with a local workdir, gitcollector is started and then it fails trying to get org repos.

$ GITCOLLECTOR_LIBRARY=/tmp/gitcollector GITHUB_ORGANIZATIONS= gitcollector download
WARN GET https://api.github.com/orgs//repos?per_page=100: 404 Not Found []
INFO collection finished in 436.35442ms

imo, if no org is passed, it should exit 0 with a warning, e.g. no organizations to scrape instead of trying to get repos from a no-org