Add sce

[rikhuijzer]

Show code and error

  function get_error(expr::String)
      try
          sco(expr)
      catch e
          exc, bt = last(Base.catch_stack())
          stacktrace = sprint(Base.showerror, exc, bt)::String
          stacktrace = Books.clean_stacktrace(stacktrace)
          lines = split(stacktrace, '\n')
          lines = lines[1:end-8]
          join(lines, '\n')
      end
  end

  function show_error(expr::String)
      out = output_block(JDS.get_error(expr))
      code = code_block(expr)
      """
      $code
      $out
      """
  end

Also here, we need a process and post. The latter is useful in shortening output.