Showing Validation.Errors in tooltip not working

Question

I'm trying to show the Validation.error message inside a tooltip in my DataGrid cell but without success. Here is the code of the concerned column. Please tell me if i'm doing something wrong. I did get the idea of the converter from this tutorial

 <DataGridTemplateColumn Width="200" Header="Numéro Chassis" >
                            <DataGridTemplateColumn.CellTemplate>
                                <DataTemplate>
                                    <TextBlock Text="{Binding NumeroChassis}">
                                    </TextBlock>
                                </DataTemplate>
                            </DataGridTemplateColumn.CellTemplate>
                            <DataGridTemplateColumn.CellEditingTemplate>
                                <DataTemplate>
                                    <TextBox ToolTip="{Binding Path=(Validation.Errors),
                             Converter={StaticResource eToMConverter}}">
                                        <TextBox.Text>
                                            <Binding Path="NumeroChassis" >
                                                <Binding.ValidationRules>
                                                    <inf:NoNullValidationRule/>
                                                </Binding.ValidationRules>
                                            </Binding>
                                        </TextBox.Text>
                                    </TextBox>
                                </DataTemplate>
                            </DataGridTemplateColumn.CellEditingTemplate>
                        </DataGridTemplateColumn>

Answer 1

I did work with RelativeSource and it worked pretty well, just replacing :

<TextBox ToolTip="{Binding Path=(Validation.Errors),
                             Converter={StaticResource eToMConverter}}">

with :
<TextBox ToolTip="{Binding Path=(Validation.Errors), RelativeSource={RelativeSource Self},Converter={StaticResource eToMConverter}}">