def error
      @values.each_pair{|observable, value|
        return Error.new(observable, value) if Exception === value
      }
      return nil
    end
def error
      @values.each_pair{|observable, value|
        return Error.new(observable, value) if Exception === value
      }
      return nil
    end
Return Error for first crashed observable.

 Should only be used if error?

 @return [Exception, nil]