Log failure reason when trying DNS managers

[hulthe]

Might make it a bit easier to diagnose DNS issues on linux.

Example output:

[2024-11-28 11:25:05.950][talpid_core::dns][INFO] Setting DNS servers: Tunnel DNS: {10.64.0.1} Non-tunnel DNS: {}
[2024-11-28 11:25:05.951][talpid_core::dns::imp][DEBUG] Can't manage DNS using systemd-resolved: systemd-resolved operation failed
[2024-11-28 11:25:05.952][talpid_dbus::network_manager][DEBUG] /etc/resolv.conf differs from reference resolv.conf, therefore NM is not managing DNS
[2024-11-28 11:25:05.952][talpid_core::dns::imp][DEBUG] Can't manage DNS using network manager: NetworkManager is not managing DNS
[2024-11-28 11:25:05.952][talpid_core::dns::imp][DEBUG] Can't manage DNS using resolveconf: Failed to detect 'resolvconf' program
[2024-11-28 11:25:05.952][talpid_core::dns::imp::static_resolv_conf][DEBUG] No DNS state backup to restore
[2024-11-28 11:25:05.952][talpid_core::dns::imp][DEBUG] Managing DNS via /etc/resolv.conf

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This change is 

[dlon]

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