RFR: git-pr: publish later and polish error messages

[Erik Helin]

Hi all,

please review this patch that ensure that `git pr create` runs `git publish` as
_late_ as possible. This is to avoid GitHub showing the "You just pushed a
branch" notification while the user is writing the pull request message in the
editor.

I also polished several of the error messages to provide better info and do
better checks.

Testing:
- manual testing of `git pr create` on Linux x64

Thanks,
Erik

-------------

Commits:
 - b999c162: git-pr: publish later and polish error messages

Changes: https://git.openjdk.java.net/skara/pull/463/files
 Webrev: https://webrevs.openjdk.java.net/skara/463/webrev.00
  Stats: 86 lines in 1 file changed: 44 ins; 30 del; 12 mod
  Patch: https://git.openjdk.java.net/skara/pull/463.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/463/head:pull/463

PR: https://git.openjdk.java.net/skara/pull/463