Я уже сделал то, что хотел, это было так:
<Style TargetType="{x:Type TextBox}">
<Setter Property="Validation.ErrorTemplate">
<Setter.Value>
<ControlTemplate>
<Grid>
<Polygon Points="0,0 10,0 0,10 0,0" HorizontalAlignment="Right" Fill="Red" FlowDirection="RightToLeft"></Polygon>
<Border BorderBrush="Red" BorderThickness="1">
<AdornedElementPlaceholder Name="controlWithError" />
</Border>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
<Style.Triggers>
<Trigger Property="Validation.HasError" Value="true">
<Setter Property="ToolTip" Value="{Binding RelativeSource={x:Static RelativeSource.Self}, Path=(Validation.Errors)[0].ErrorContent}"/>
</Trigger>
</Style.Triggers>
</Style>