add example of datavalidation on autocomplete box.

samples/ControlCatalog/Pages/AutoCompleteBoxPage.xaml

@@ -1,5 +1,6 @@
 <UserControl xmlns="https://github.com/avaloniaui"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:sys="clr-namespace:System;assembly=netstandard"
              x:Class="ControlCatalog.Pages.AutoCompleteBoxPage">
   <StackPanel Orientation="Vertical" Spacing="4">
     <TextBlock Classes="h1">AutoCompleteBox</TextBlock>
@@ -56,6 +57,16 @@
                          Width="200"
                          Margin="0,0,0,8"
                          FilterMode="None"/>
+        
+        <TextBlock Text="With Validation Errors"/>
+        <AutoCompleteBox Name="ValidationErrors"
+                         Width="200"
+                         Margin="0,0,0,8"
+                         FilterMode="None">
+          <DataValidationErrors.Error>
+            <sys:Exception /> 
+          </DataValidationErrors.Error>
+        </AutoCompleteBox>
       </StackPanel>
     </StackPanel>
   </StackPanel>