rpc: Actually throw help when passed invalid number of params

Summary:

• rpc: Add RPCHelpMan::IsValidNumArgs()

This is a backport of Core PR15401

Depends on D6064

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6220