Pull request: all: imp code, err handling

Closes #2571.

Squashed commit of the following:

commit a5b50ee011a995f4ab3d93314acd6f0ca82d99cf
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Mar 15 14:05:25 2021 +0300

    all: imp code

commit bc610f8f438549e8c6b04c8a213b5422dda2aff5
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Mar 12 20:00:14 2021 +0300

    all: imp code, err handling

internal/querylog/qlogreader.go

@@ -4,8 +4,10 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"os"
 
 	"github.com/AdguardTeam/AdGuardHome/internal/agherr"
+	"github.com/AdguardTeam/golibs/log"
 )
 
 // QLogReader allows reading from multiple query log files in the reverse order.
@@ -30,8 +32,16 @@ func NewQLogReader(files []string) (*QLogReader, error) {
 	for _, f := range files {
 		q, err := NewQLogFile(f)
 		if err != nil {
-			// Close what we've already opened
-			_ = closeQFiles(qFiles)
+			if errors.Is(err, os.ErrNotExist) {
+				continue
+			}
+
+			// Close what we've already opened.
+			cerr := closeQFiles(qFiles)
+			if cerr != nil {
+				log.Debug("querylog: closing files: %s", cerr)
+			}
+
 			return nil, err
 		}