add pack/unpack errors

errors.go

@@ -0,0 +1,28 @@
+package iso8583
+
+// UnpackError returns error with possibility to access RawMessage when
+// connection failed to unpack message
+type UnpackError struct {
+	Err        error
+	RawMessage []byte
+}
+
+func (e *UnpackError) Error() string {
+	return e.Err.Error()
+}
+
+func (e *UnpackError) Unwrap() error {
+	return e.Err
+}
+
+type PackError struct {
+	Err error
+}
+
+func (e *PackError) Error() string {
+	return e.Err.Error()
+}
+
+func (e *PackError) Unwrap() error {
+	return e.Err
+}

message.go

@@ -126,7 +126,18 @@ func (m *Message) GetFields() map[int]field.Field {
 	return fields
 }
 
+// Pack returns the packed message or an error if the message is invalid
+// error is of type *PackError
 func (m *Message) Pack() ([]byte, error) {
+	data, err := m.pack()
+	if err != nil {
+		return nil, &PackError{Err: err}
+	}
+
+	return data, nil
+}
+
+func (m *Message) pack() ([]byte, error) {
 	packed := []byte{}
 	m.Bitmap().Reset()
 
@@ -160,7 +171,19 @@ func (m *Message) Pack() ([]byte, error) {
 	return packed, nil
 }
 
+// Unpack unpacks the message from the given byte slice or returns an error
+// which is of type *UnpackError and contains the raw message
 func (m *Message) Unpack(src []byte) error {
+	if err := m.unpack(src); err != nil {
+		return &UnpackError{
+			Err:        err,
+			RawMessage: src,
+		}
+	}
+	return nil
+}
+
+func (m *Message) unpack(src []byte) error {
 	var off int
 
 	// reset fields that were set

message_test.go

@@ -535,12 +535,26 @@ func TestPackUnpack(t *testing.T) {
 		assert.Equal(t, "Another test text", data.F120.Value())
 	})
 
+	t.Run("Pack invalid message returns error of *PackError type", func(t *testing.T) {
+		message := NewMessage(spec)
+		message.MTI("1")
+
+		_, err := message.Pack()
+		require.Error(t, err)
+
+		var packErr *PackError
+		require.ErrorAs(t, err, &packErr)
+	})
+
 	t.Run("Unpack nil", func(t *testing.T) {
 		message := NewMessage(spec)
 
 		err := message.Unpack(nil)
 
 		require.Error(t, err)
+
+		var unpackError *UnpackError
+		require.ErrorAs(t, err, &unpackError)
 	})
 
 	t.Run("Unpack short mti", func(t *testing.T) {
@@ -551,6 +565,10 @@ func TestPackUnpack(t *testing.T) {
 		err := message.Unpack([]byte(rawMsg))
 
 		require.Error(t, err)
+
+		var unpackError *UnpackError
+		require.ErrorAs(t, err, &unpackError)
+		require.Equal(t, rawMsg, unpackError.RawMessage)
 	})
 
 	// this test should check that BCD fields are packed and