prometheus_sysctl_exporter: ignore ENOENT for mibs specified on the CLI

They might belong to kernel modules not currently loaded, or to other
kernel versions. Ignoring them allows the configuration to be shared
between multiple hosts.

Sponsored by: Axcient
Reviewed by: rew
Differential Revision: https://reviews.freebsd.org/D35540

(cherry picked from commit 982f980b86d6bab4d55452d17bf3f5eb04e5f01e)