bench: Don't return a bool from main

Summary:

Return `EXIT_SUCCESS` from `main()` on error, not the bool `false`
(introduced in #13112). This is the correct value to return on error,
and also shuts up a clang warning.

Also add a final return for clarity.

Backport of core PR13349
https://github.com/bitcoin/bitcoin/pull/13349/files

Test Plan:
With clang:

ninja bench-bitcoin

Ensure there is no -Wmain warning during the build.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3785