Unpack AggregateException in TextBox.

src/Avalonia.Controls/TextBox.cs

@@ -495,11 +495,17 @@ namespace Avalonia.Controls
                 else
                 {
                     classes.Add(":error");
-                    DataValidationErrors = new[] { status.Error };
+                    DataValidationErrors = UnpackException(status.Error);
                 }
             }
         }
 
+        private static IEnumerable<Exception> UnpackException(Exception exception)
+        {
+            var aggregate = exception as AggregateException;
+            return aggregate == null ? Enumerable.Repeat(exception, 1) : aggregate.InnerExceptions;
+        }
+
         private int CoerceCaretIndex(int value)
         {
             var text = Text;