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/aghtest/aghtest.go

@@ -30,7 +30,7 @@ func ReplaceLogWriter(t *testing.T, w io.Writer) {
 
 // ReplaceLogLevel sets logging level to l and uses Cleanup method of t to
 // revert changes.
-func ReplaceLogLevel(t *testing.T, l int) {
+func ReplaceLogLevel(t *testing.T, l log.Level) {
 	switch l {
 	case log.INFO, log.DEBUG, log.ERROR:
 		// Go on.

internal/aghtest/upstream.go

@@ -168,9 +168,6 @@ type TestErrUpstream struct {
 
 // Exchange always returns nil Msg and non-nil error.
 func (u *TestErrUpstream) Exchange(*dns.Msg) (*dns.Msg, error) {
-	// We don't use an agherr.Error to avoid the import cycle since aghtests
-	// used to provide the utilities for testing which agherr (and any other
-	// testable package) should be able to use.
 	return nil, fmt.Errorf("errupstream: %w", u.Err)
 }