How to use validators to display multiple Errors

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WPF

C#5.0

Hey there,
long time since i asked a question. I want to display all validation-errors and disable two buttons addicted from the validation-error.

Background:
I have a table where the user can add and delete rows. The primerykey (Number)
shall be validated and if there is an error the add or remove button have to be disabled.
1: Error -> Key-Exsists -> Disable add button; enable remove button
2: Error -> Key is used in other table -> disable remove button

So it's possible that both errors appear at the same time. Therefor i need some help.
I use the ValidationList of Rohit Dot net: WPF - Multiple Validation on Single Control[^]

My error-template looks like:

 <ControlTemplate 
                x:Key="validationErrorTemplate">
                <DockPanel>
                    <StackPanel 
                        Orientation="Horizontal" 
                        DockPanel.Dock="Top">
                        <Grid 
                            Width="12" 
                            Height="12">
                            <Ellipse 
                                Width="12" 
                                Height="12"
                                Fill="Red" 
                                HorizontalAlignment="Center"
                                VerticalAlignment="Center"/>
                            <TextBlock 
                                Foreground="White"
                                FontWeight="Heavy"
                                FontSize="8" 
                                HorizontalAlignment="Center"
                                VerticalAlignment="Center" 
                                TextAlignment="Center"
                                ToolTip="{Binding Path=ErrorContent}" />
                        </Grid>
                        <TextBlock 
                            Text="{Binding                                        
                                    Path=ErrorContent,
                                    UpdateSourceTrigger=PropertyChanged,
                                    ValidatesOnDataErrors=True,
                                    ValidatesOnExceptions=True}"  
                   FontSize="12"  
                   Foreground="Red"  
                   Margin="5 0 0 0"/>
                    </StackPanel>
                    <AdornedElementPlaceholder
                        x:Name="ErrorAdorner" />
                </DockPanel>
</ControlTemplate>

I want something like a converter which has the validationerrors as input and return them gathered as string. Additional i need some mechanism to disable the both buttons. Momentary i use Button-Styles for that:

<Style 
                x:Key="PopUpAddButtonStyle"
                BasedOn="{StaticResource {x:Type Button}}"
                TargetType="{x:Type Button}">
                <Setter Property="IsEnabled" Value="False" />
                <Style.Triggers>
                    <MultiDataTrigger>
                        <MultiDataTrigger.Conditions>
                            <Condition Binding="{Binding ElementName=TextBoxA, Path=(Validation.HasError)}" Value="false"/>
                            <Condition Binding="{Binding ElementName=TextBoxB, Path=(Validation.HasError)}" Value="false" />
                            <Condition Binding="{Binding ElementName=TextBoxC, Path=(Validation.HasError)}" Value="false" />
                            <Condition Binding="{Binding ElementName=TextBoxD, Path=(Validation.HasError)}" Value="false" />
                            <Condition Binding="{Binding ElementName=TextBoxE, Path=(Validation.HasError)}" Value="false" />
                        </MultiDataTrigger.Conditions>
                        <Setter Property="IsEnabled" Value="True" />
                    </MultiDataTrigger>
                </Style.Triggers>
            </Style>

Thanks for any help.
Greetings 3DC#