Improve error messages for invalid named colors

[hx2A]

Discussion: #525

Two things need to happen:

• py5.css4_colors and py5.xkcd_colors need to use __getattr__() or something to capture invalid colors and give a better response
• The _hex_converter() function in decorators.py need to throw a new exception when a string argument is not found instead of returning None

[hx2A]

@villares , what do you think of the following exception messages?

In [3]: py5.run_sketch()              

py5 encountered an error in your code:                 
                                                                    
File "<ipython-input-2-e850e1b50258>", line 3, in setup
    1    def setup():                            
    2        py5.fill('red')                          
--> 3        py5.stroke('asdfasdf')             
                                                                                                                                        
RuntimeError: unknown color or unparsable color string 'asdfasdf'

and

In [6]: py5.run_sketch()

py5 encountered an error in your code:

File "<ipython-input-5-91963aa97d2e>", line 3, in setup
    1    def setup():
    2        py5.fill('red')
--> 3        py5.stroke(py5.xkcd_colors.asdfasdf)
    ..................................................
     py5.xkcd_colors.asdfasdf = # AttributeError
          py5.xkcd_colors = <module 'py5_tools.colors.xkcd' from '/hom
           e/jim/INSTALL/anaconda3/envs/py5/lib/python3.9/site-packages
           /py5_tools/colors/xkcd.py'>
    ..................................................

AttributeError: unknown xkcd color 'asdfasdf'

?

[villares]

looks great! could you raise ValueError?