Added index of error to LexError

[pydanny]

This allows for easier writing of tools empowered by the sly library. Now you can extend the library to get a better context of the lexed data.

Working example taken from the TSD parser:

for path in glob('chapters/*'):
    with open(path) as f:
        data = f.read()
    try:
        tokens = [x for x in tokenize(data)]
    except LexError as err:
        print(f"ERROR AT {err.error_index} in {path}")
        print('*'*40)
        res = data[err.error_index-20:err.error_index+20]
        print(res)
        print('*'*40)
        # print("Precise:" + data[err.error_index-1:err.error_index+1], sep='')
        break