tiny refactor for ArgsManager
This commit contains 2 refactors.
- mark "const" on ArgsManager::GetHelpMessage and IsArgKnown.
- remove unused "error" argument from ArgsManager::IsArgKnown.
Firstly, I mark "const" on where it is possible to. It is mentioned
before (e.g. https://github.com/bitcoin/bitcoin/pull/13190#pullrequestreview-118823133).
And about 2nd change, ArgsManager::IsArgKnown was added at commit #4f8704d which was
merged at PR #13112. But from its beggining, "error" argument never be used.
I think it should be refactored.
This is a backport of Core PR13733
Reviewers: #bitcoin_abc, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D4168