message: Lock queue if updating toolchain fails
  Reduces the impact from failures like rather than continuing to
  build crates with the failing toolchain we will just stop building
  crates until an admin has time to investigate the cause.
diff: @@ -87,6 +87,12 @@ impl DocBuilder {
  .map(|r| PackageKind::Registry(r.as_str()))
  .unwrap_or(PackageKind::CratesIo);
  + if let Err(err) = builder.update_toolchain() {
  + log::error!("Updating toolchain failed, locking queue: {}", err);
  + self.lock()?;
  + return Err(err);
  + }
  +
  builder.build_package(&krate.name, &krate.version, kind)?;
  Ok(())
  })?;
  