tiny refactor for ArgsManager

Summary:
This commit contains 2 refactors.

1. mark "const" on ArgsManager::GetHelpMessage and IsArgKnown.
2. 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

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4168