Pull request: replace agherr with golibs' errors

Merge in DNS/adguard-home from golibs-errors to master

Squashed commit of the following:

commit 5aba278a31c5a213bd9e08273ce7277c57713b22
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon May 24 17:05:18 2021 +0300

    all: imp code

commit f447eb875b81779fa9e391d98c31c1eeba7ef323
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon May 24 15:33:45 2021 +0300

    replace agherr with golibs' errors

internal/dhcpd/db.go

@@ -4,11 +4,11 @@ package dhcpd
 
 import (
 	"encoding/json"
-	"errors"
 	"net"
 	"os"
 	"time"
 
+	"github.com/AdguardTeam/golibs/errors"
 	"github.com/AdguardTeam/golibs/log"
 	"github.com/google/renameio/maybe"
 )