Skip setup `tempbuf.el` if unreachable to emacswiki.org

[MinoruSekine]

• Downloading from emacswiki.org randomly fails on GitHub Actions workflow
• Root cause and/or solution for above have not been found
  • It can't solve to use curl instead of url-copy-file
• So tentative but non-short-term solution is necessary
  • Handle download error explicitly
  • If flag like my-ignore-download-error-from-emacswiki is set, ignore above error and skip setup for those elisp