add some tips for BadCredentialsException

[Lslightly]

Since #1753 has so many comments about BadCredentialsException, is it possible to add some tips when raising the exception? For example, add a link to the issue or suggest some checks like #1753 (comment).

I think this saves users' time since searching for Bad Credentials is time-consuming and reading such long issue without an conclusion is tiring and depressing.

I guess there are many downstream apps that uses cache or other persistent system like database because getting issues/comments or other information from github is time-consuming. So I think it is important to add some tips or notes when raising BadCredentialsException

Besides, it is better to add a function check to check the validity of objects and a function refresh for re-validate a object since most fields of a Issue-like object is valid except the token field.