class_docstr: KeyErrors are not printed as beautifully as other errors (this is so that
  {}[''] prints    "KeyError: ''" and not "KeyError:"). So here we use
  LookupError's __str__ to make a PlotlyKeyError object which will print nicer
  error messages for KeyErrors.
code: def __str__(self):
      return LookupError.__str__(self)
  
label_desc: Use image node_id 1 for calling the PlotlyKeyError obj's underlying member method code with example usage: obj.__str__() and returns: LookupError