message: Improve unknown alias error message
diff: @@ -95,11 +95,9 @@ fn read_recipients(
  eprintln!("Warning: duplicate {}", arg);
  } else {
  // Replace the alias in the arguments list with its expansion
  - arguments.extend(
  - aliases.remove(&arg[ALIAS_PREFIX.len()..]).ok_or_else(|| {
  - io::Error::new(io::ErrorKind::InvalidInput, "unknown alias")
  - })?,
  - );
  + arguments.extend(aliases.remove(&arg[ALIAS_PREFIX.len()..]).ok_or_else(|| {
  + io::Error::new(io::ErrorKind::InvalidInput, format!("unknown {}", arg))
  + })?);
  seen_aliases.push(arg);
  }
  } else if arg.starts_with(GITHUB_PREFIX) {
  