Make SSH_FXP_STATUS "message" field optional.

Cisco SSH 2.0 server doesn't fill out the "message" field,
even though it returns valid status codes. This makes
File.Read method error out without actually reading the
file due to unmarshalStatus returning a "packet too short"
error.

client.go

@@ -1094,10 +1094,7 @@ func unmarshalStatus(id uint32, data []byte) error {
 		return &unexpectedIDErr{id, sid}
 	}
 	code, data := unmarshalUint32(data)
-	msg, data, err := unmarshalStringSafe(data)
-	if err != nil {
-		return err
-	}
+	msg, data, _ := unmarshalStringSafe(data)
 	lang, _, _ := unmarshalStringSafe(data)
 	return &StatusError{
 		Code: code,

client_test.go

@@ -116,7 +116,9 @@ func TestUnmarshalStatus(t *testing.T) {
 			desc:   "missing error message and language tag",
 			reqID:  1,
 			status: idCode,
-			want:   errShortPacket,
+			want: &StatusError{
+				Code: ssh_FX_FAILURE,
+			},
 		},
 		{
 			desc:   "missing language tag",