git reviewrequest crashes with '422 Unprocessable entity' if PR already exists

[benklop]

If the PR has already been created, for example by using the createpr command, scgitx will crash while trying to create the PR, instead of using the existing one.

11:41 $ git reviewrequest 
updating opv-915-reboot-check to have most recent changes from master

$ git pull origin opv-915-reboot-check
From https://github.com/socialcast/scmc
 * branch            opv-915-reboot-check -> FETCH_HEAD
Already up-to-date.

$ git pull origin master
From https://github.com/socialcast/scmc
 * branch            master     -> FETCH_HEAD
Already up-to-date.

$ git push origin HEAD
Everything up-to-date
Creating pull request for opv-915-reboot-check against master in socialcast/scmc
GitHub request failed: Validation Failed
/home/benklop/.rvm/gems/ruby-2.2.4@scmc/gems/socialcast-git-extensions-4.0.2/lib/socialcast-git-extensions/github.rb:128:in `throw': uncaught throw 422 Unprocessable Entity: {"message":"Validation Failed","errors":[{"resource":"PullRequest","code":"custom","message":"A pull request already exists for socialcast:opv-915-reboot-check."}],"documentation_url":"https://developer.github.com/v3/pulls/#create-a-pull-request"} (UncaughtThrowError)
    from /home/benklop/.rvm/gems/ruby-2.2.4@scmc/gems/socialcast-git-extensions-4.0.2/lib/socialcast-git-extensions/github.rb:128:in `rescue in github_api_request'
    from /home/benklop/.rvm/gems/ruby-2.2.4@scmc/gems/socialcast-git-extensions-4.0.2/lib/socialcast-git-extensions/github.rb:124:in `github_api_request'
    from /home/benklop/.rvm/gems/ruby-2.2.4@scmc/gems/socialcast-git-extensions-4.0.2/lib/socialcast-git-extensions/github.rb:43:in `create_pull_request'
    from /home/benklop/.rvm/gems/ruby-2.2.4@scmc/gems/socialcast-git-extensions-4.0.2/lib/socialcast-git-extensions/cli.rb:39:in `createpr'
    from /home/benklop/.rvm/gems/ruby-2.2.4@scmc/gems/socialcast-git-extensions-4.0.2/lib/socialcast-git-extensions/cli.rb:98:in `reviewrequest'
    from /home/benklop/.rvm/gems/ruby-2.2.4@scmc/gems/thor-0.19.1/lib/thor/command.rb:27:in `run'
    from /home/benklop/.rvm/gems/ruby-2.2.4@scmc/gems/thor-0.19.1/lib/thor/invocation.rb:126:in `invoke_command'
    from /home/benklop/.rvm/gems/ruby-2.2.4@scmc/gems/thor-0.19.1/lib/thor.rb:359:in `dispatch'
    from /home/benklop/.rvm/gems/ruby-2.2.4@scmc/gems/thor-0.19.1/lib/thor/base.rb:440:in `start'
    from /home/benklop/.rvm/gems/ruby-2.2.4@scmc/gems/socialcast-git-extensions-4.0.2/bin/git-reviewrequest:4:in `<top (required)>'
    from /home/benklop/.rvm/gems/ruby-2.2.4@scmc/bin/git-reviewrequest:23:in `load'
    from /home/benklop/.rvm/gems/ruby-2.2.4@scmc/bin/git-reviewrequest:23:in `<main>'
    from /home/benklop/.rvm/gems/ruby-2.2.4@scmc/bin/ruby_executable_hooks:15:in `eval'
    from /home/benklop/.rvm/gems/ruby-2.2.4@scmc/bin/ruby_executable_hooks:15:in `<main>'