Change log implementation to return the custom error.

internal/log/log.go

@@ -1,7 +1,6 @@
 package log
 
 import (
-	"fmt"
 	"io"
 	"io/ioutil"
 	"os"
@@ -118,7 +117,7 @@ func (l *Log) Read(off uint64) (*api.Record, error) {
 	// The condition after the of shouldn't matter because we check for this in the for.
 	// The code comes from the book.
 	if s == nil || s.nextOffset <= off {
-		return nil, fmt.Errorf("offset out of range: %d", off)
+		return nil, api.ErrOffsetOutOfRange{Offset: off}
 	}
 	return s.Read(off)
 }

internal/log/log_test.go

@@ -53,7 +53,8 @@ func testAppendRead(t *testing.T, log *Log) {
 func testOutOfRangeErr(t *testing.T, log *Log) {
 	read, err := log.Read(1)
 	require.Nil(t, read)
-	require.Error(t, err)
+	apiErr := err.(api.ErrOffsetOutOfRange)
+	require.Equal(t, uint64(1), apiErr.Offset)
 }
 
 func testInitExisting(t *testing.T, o *Log) {