chore: make error clearer (#1514)

core/filex/rangereader.go

@@ -5,6 +5,9 @@ import (
 	"os"
 )
 
+// errExceedFileSize indicates that the file size is exceeded.
+var errExceedFileSize = errors.New("exceed file size")
+
 // A RangeReader is used to read a range of content from a file.
 type RangeReader struct {
 	file  *os.File
@@ -29,7 +32,7 @@ func (rr *RangeReader) Read(p []byte) (n int, err error) {
 	}
 
 	if rr.stop < rr.start || rr.start >= stat.Size() {
-		return 0, errors.New("exceed file size")
+		return 0, errExceedFileSize
 	}
 
 	if rr.stop-rr.start < int64(len(p)) {