def to_s
      super + ":\n" +
      format_observation(result.control) + "\n" +
      result.candidates.map { |candidate| format_observation(candidate) }.join("\n") +
      "\n"
    end
def to_s
      super + ":\n" +
      format_observation(result.control) + "\n" +
      result.candidates.map { |candidate| format_observation(candidate) }.join("\n") +
      "\n"
    end
The default formatting is nearly unreadable, so make it useful.

 The assumption here is that errors raised in a test environment are
 printed out as strings, rather than using #inspect.