[CMAKE] Silent the man pages generation

Summary:
This diff makes the man pages generation silent, unless there is a
failure.

Depends on D7473.

Test Plan:

ninja install-manpages

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7470