Maint: pass only single strings to logger methods

- Do not assume formatting from logger's interface
- Allow to change golibs in the future to accept only strings for logger methods

internal/firewall/vpn.go

@@ -26,7 +26,7 @@ func (c *configurator) SetVPNConnection(ctx context.Context, connection models.O
 	remove := true
 	if c.vpnConnection.IP != nil {
 		if err := c.acceptOutputTrafficToVPN(ctx, c.defaultInterface, c.vpnConnection, remove); err != nil {
-			c.logger.Error("cannot remove outdated VPN connection through firewall: %s", err)
+			c.logger.Error("cannot remove outdated VPN connection through firewall: " + err.Error())
 		}
 	}
 	c.vpnConnection = models.OpenVPNConnection{}