Graceful error if the source or target repo doesn't exist

[antislice]

I just tried to move things from codeforamerica/NOLA instead of codeforamerica/nola-2016-fellows and got this:

/Users/georgiaandrews/.rvm/gems/ruby-2.2.2@github2github/gems/octokit-4.3.0/lib/octokit/response/raise_error.rb:16:in `on_complete': GET https://api.github.com/repos/codeforamerica/NOLA/issues?labels=job-seekers&state=open: 404 - Not Found // See: https://developer.github.com/v3 (Octokit::NotFound)
    from /Users/georgiaandrews/.rvm/gems/ruby-2.2.2@github2github/gems/faraday-0.9.2/lib/faraday/response.rb:9:in `block in call'
    from /Users/georgiaandrews/.rvm/gems/ruby-2.2.2@github2github/gems/faraday-0.9.2/lib/faraday/response.rb:57:in `on_complete'
    from /Users/georgiaandrews/.rvm/gems/ruby-2.2.2@github2github/gems/faraday-0.9.2/lib/faraday/response.rb:8:in `call'
    from /Users/georgiaandrews/.rvm/gems/ruby-2.2.2@github2github/gems/octokit-4.3.0/lib/octokit/middleware/follow_redirects.rb:73:in `perform_with_redirection'
    from /Users/georgiaandrews/.rvm/gems/ruby-2.2.2@github2github/gems/octokit-4.3.0/lib/octokit/middleware/follow_redirects.rb:61:in `call'
    from /Users/georgiaandrews/.rvm/gems/ruby-2.2.2@github2github/gems/faraday-0.9.2/lib/faraday/rack_builder.rb:139:in `build_response'
    from /Users/georgiaandrews/.rvm/gems/ruby-2.2.2@github2github/gems/faraday-0.9.2/lib/faraday/connection.rb:377:in `run_request'
    from /Users/georgiaandrews/.rvm/gems/ruby-2.2.2@github2github/gems/faraday-0.9.2/lib/faraday/connection.rb:140:in `get'
    from /Users/georgiaandrews/.rvm/gems/ruby-2.2.2@github2github/gems/sawyer-0.7.0/lib/sawyer/agent.rb:94:in `call'
    from /Users/georgiaandrews/.rvm/gems/ruby-2.2.2@github2github/gems/octokit-4.3.0/lib/octokit/connection.rb:154:in `request'
    from /Users/georgiaandrews/.rvm/gems/ruby-2.2.2@github2github/gems/octokit-4.3.0/lib/octokit/connection.rb:84:in `paginate'
    from /Users/georgiaandrews/.rvm/gems/ruby-2.2.2@github2github/gems/octokit-4.3.0/lib/octokit/client/issues.rb:32:in `list_issues'
    from /Users/georgiaandrews/.rvm/gems/ruby-2.2.2@github2github/gems/octokit-4.3.0/lib/OctoKit.rb:46:in `method_missing'
    from move_issues.rb:64:in `<main>'