bench: Represents paths with fs::path instead of std::string

Summary:
Also uses fs::path quoting in bench printed strings and fixes a
misleading error message.

Originally suggested https://github.com/bitcoin/bitcoin/pull/20744#issuecomment-1022486215

Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>

This is a backport of core#24252

Test Plan: ninja bench-bitcoin

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15063