Pull request: 2546 updater fix

Merge in DNS/adguard-home from 2546-updater-fix to master

Closes #2546.

Squashed commit of the following:

commit af243c9fad710efe099506fda281e628c3e5ec30
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed Jan 13 14:33:37 2021 +0300

    updater: fix go 1.14 compat

commit 742fba24b300ce51c04acb586996c3c75e56ea20
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed Jan 13 13:58:27 2021 +0300

    util: imp error check

commit c2bdbce8af657a7f4b7e05c018cfacba86e06753
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Jan 11 18:51:26 2021 +0300

    all: fix and refactor update checking

internal/util/helpers.go

@@ -26,7 +26,7 @@ func ContainsString(strs []string, str string) bool {
 // FileExists returns true if file exists.
 func FileExists(fn string) bool {
 	_, err := os.Stat(fn)
-	return err == nil
+	return err == nil || !os.IsNotExist(err)
 }
 
 // RunCommand runs shell command.