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/util/helpers.go

@@ -8,29 +8,12 @@ import (
 	"bytes"
 	"fmt"
 	"io/ioutil"
-	"os"
 	"os/exec"
 	"path/filepath"
 	"runtime"
 	"strings"
 )
 
-// ContainsString checks if string is in the slice of strings.
-func ContainsString(strs []string, str string) bool {
-	for _, s := range strs {
-		if s == str {
-			return true
-		}
-	}
-	return false
-}
-
-// FileExists returns true if file exists.
-func FileExists(fn string) bool {
-	_, err := os.Stat(fn)
-	return err == nil || !os.IsNotExist(err)
-}
-
 // RunCommand runs shell command.
 func RunCommand(command string, arguments ...string) (int, string, error) {
 	cmd := exec.Command(command, arguments...)