failed to create pull request: repeat PR, better error msging

Got this error code, it would be great if it was formatted a bit better and provided a link to the existing pull request which already exists (so I can go take a look at that one and confirm)

No file or test is named in the issue. Start by locating the pull-request creation entry point and the error path for an existing pull request; confirm how that existing PR is represented. Done means the repeat-PR failure is clearer and includes a usable link to the existing pull request, with coverage for the updated message.

索引モデルが issue の本文から書いたものです。