Pull request: * querylog: fix end of log handling

Merge in DNS/adguard-home from 2229-query-log to master

Closes #2229.

Squashed commit of the following:

commit 32508a3f3b1e098869e1649a2774f1f17d14d41f
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Nov 10 16:51:25 2020 +0300

    * querylog: add test

commit 774159cc313a0284a8bb8327489671e5d7a3e4eb
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Nov 10 15:26:26 2020 +0300

    * querylog: better errors

commit 27b13a4dcaff9e8f9b08aec81c0c03f62ebd3fa5
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Nov 10 15:18:51 2020 +0300

    * querylog: fix end of log handling

internal/agherr/agherr.go

@@ -7,6 +7,14 @@ import (
 	"strings"
 )
 
+// Error is the constant error type.
+type Error string
+
+// Error implements the error interface for Error.
+func (err Error) Error() (msg string) {
+	return string(err)
+}
+
 // manyError is an error containing several wrapped errors.  It is created to be
 // a simpler version of the API provided by github.com/joomcode/errorx.
 type manyError struct {