fusefrontend: print file hexdump on header error

This should help debugging https://github.com/rfjakob/gocryptfs/issues/363 ,
but does no harm in normal operation as it only prints ciphertext to the log.

internal/fusefrontend/file.go

@@ -4,6 +4,7 @@ package fusefrontend
 
 import (
 	"bytes"
+	"encoding/hex"
 	"fmt"
 	"io"
 	"log"
@@ -158,7 +159,12 @@ func (f *File) doRead(dst []byte, off uint64, length uint64) ([]byte, fuse.Statu
 				// Empty file
 				return nil, fuse.OK
 			}
-			tlog.Warn.Printf("doRead %d: corrupt header: %v", f.qIno.Ino, err)
+			buf := make([]byte, 100)
+			n, _ := f.fd.ReadAt(buf, 0)
+			buf = buf[:n]
+			hexdump := hex.EncodeToString(buf)
+			tlog.Warn.Printf("doRead %d: corrupt header: %v\nFile hexdump (%d bytes): %s",
+				f.qIno.Ino, err, n, hexdump)
 			return nil, fuse.EIO
 		}
 		// Save into the file table