After some thinking I realized that there is indeed a continuum between "the exception that prove the existence of a rule" and "the exception that proves the correctness/validity of a rule" and in this post it is unclear which meaning was intended.
Anyway I believe my point still applies to most common usages.
The phrase "the exception that proves the rule" is meant to say one of:
1) this exception shows that the rule is needed, as in "the exception that proves the need of a rule"
2) if a case is extremely execptional then there must be a normality which is the rule, as in "the exception that proves the existence of the rule"
There is no authority argument here, I simply like it best this way.