[notpeter]

Remove custom Errors for calls which are actually infallible.

• None of the RfdDocument mutators could actually fail.
  Markdown used type Error = ()
  Asciidoc errors were only for regex compilation failure.
• Switch to using regex!, drop type Error from the trait;
  mutators now return &mut Self.
• Remove error types:
  • RfdAsciidocError
  • RfdContentError
  • GitHubError::InvalidContent
  • RenderableRfdError::InvalidRfdContent
  • UpdateRfdContentError::InvalidContent
  • RfdError::Asciidoc
  • RfdUpdaterError::ContentOfExistingRfd
• Remove "Update would result in a malformed RFD" 400 from set-attribute endpoint.
  It was unreachable.
• Make some regexes compile once with regex! (infallible) instead of at runtime