Use a custom exception ValueError subclass for the special tokens warning

[simonw]

This code here:

tiktoken/tiktoken/core.py

Lines 375 to 383 in 39f29ce

	def raise_disallowed_special_token(token: str) -> NoReturn:
	raise ValueError(
	f"Encountered text corresponding to disallowed special token {token!r}.\n"
	"If you want this text to be encoded as a special token, "
	f"pass it to `allowed_special`, e.g. `allowed_special={{{token!r}, ...}}`.\n"
	f"If you want this text to be encoded as normal text, disable the check for this token "
	f"by passing `disallowed_special=(enc.special_tokens_set - {{{token!r}}})`.\n"
	"To disable this check for all special tokens, pass `disallowed_special=()`.\n"
	)

I wanted to do something special on this exception in my own code, so I had to write this:

try:
    tokens = encoding.encode(text, **kwargs)
except ValueError as ex:
    if 'disallowed special token' in str(ex):
        # Do something special

I suggest having a custom exception class for this instead:

class DisallowedSpecialTokenError(ValueError):
    pass

Raising that class instead would let people like me catch it explicitly, and since it's a subclass of ValueError it should not break any existing code that currently catches ValueError directly.