Use existing PR message on failure

[chrisarcand]

If something goes wrong after editing your PR message using pull-request -e (editing your message via $EDITOR), your message isn't used the next time you run the command. I do this constantly when I forget to push my changes and there's no reference to be made in the repo for the pull request, and any detailed message I wrote (which I often do) is now 'gone'.

You should be able to run the command again and the last saved reference will be used. GitHub's hub does this wonderfully.