Clarify message for the error 'pred_probs is not defined'

[jwmueller]

Currently find_label_issues docstring contains the following:

Tip: If you encounter the error 'pred_probs is not defined', try setting n_jobs = 1.

https://github.com/cleanlab/cleanlab/blob/e1d9ae4fcf95e802f4a8e112845929ee785705eb/cleanlab/filter.py#L257

Try to resolve this or provide clearer error message containing the tip when the error triggers.