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/aghio/limitedreader.go

@@ -0,0 +1,59 @@
+// Package aghio contains extensions for io package's types and methods
+package aghio
+
+import (
+	"fmt"
+	"io"
+)
+
+// LimitReachedError records the limit and the operation that caused it.
+type LimitReachedError struct {
+	Limit int64
+}
+
+// Error implements error interface for LimitReachedError.
+//
+// TODO(a.garipov): Think about error string format.
+func (lre *LimitReachedError) Error() string {
+	return fmt.Sprintf("attempted to read more than %d bytes", lre.Limit)
+}
+
+// limitedReader is a wrapper for io.Reader with limited reader and dealing with
+// errors package.
+type limitedReader struct {
+	r     io.Reader
+	limit int64
+	n     int64
+}
+
+// Read implements Reader interface.
+func (lr *limitedReader) Read(p []byte) (n int, err error) {
+	if lr.n == 0 {
+		return 0, &LimitReachedError{
+			Limit: lr.limit,
+		}
+	}
+
+	if int64(len(p)) > lr.n {
+		p = p[0:lr.n]
+	}
+
+	n, err = lr.r.Read(p)
+	lr.n -= int64(n)
+
+	return n, err
+}
+
+// LimitReader wraps Reader to make it's Reader stop with ErrLimitReached after
+// n bytes read.
+func LimitReader(r io.Reader, n int64) (limited io.Reader, err error) {
+	if n < 0 {
+		return nil, fmt.Errorf("aghio: invalid n in LimitReader: %d", n)
+	}
+
+	return &limitedReader{
+		r:     r,
+		limit: n,
+		n:     n,
+	}, nil
+}