FYI: publish: only exit on error

[Erik Helin]

Hi all,

please review this small patch that ensures that `GitPublish.main` only calls `System.exit` on error. This is needed
because `GitPrCreate.main` calls `GitPublish.main`, so `GitPublish.main` cannot unconditionally call `System.exit`.

Testing:
- [x] Manual testing of `git publish`

Thanks,
Erik

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Commit messages:
 - publish: only exit on error

Changes: https://git.openjdk.java.net/skara/pull/655/files
 Webrev: https://webrevs.openjdk.java.net/skara/655/webrev.00
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/655.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/655/head:pull/655

PR: https://git.openjdk.java.net/skara/pull/655