[X] resilience on markup (#5532)

let the previewer intercepts and recover from misformatted markups

- fixes #5508
- closes #5527

Xamarin.Forms.Xaml.UnitTests/DesignTimeLoaderTests.cs

@@ -564,23 +564,22 @@ namespace Xamarin.Forms.Xaml.UnitTests
 								<StackLayout>
 									<ListView ItemsSource=""{x:Static Foo}"" />
 									<ListView ItemsSource=""{local:Missing Test}"" />
+									<Label Text=""{Binding Foo"" />
 								</StackLayout>
 						</ContentPage>";
-
 			var exceptions = new List<Exception>();
 			Xamarin.Forms.Internals.ResourceLoader.ExceptionHandler = exceptions.Add;
 			Assert.DoesNotThrow(() => XamlLoader.Create(xaml, true));
 			Assert.That(exceptions.Count, Is.GreaterThan(1));
-    }
+		}
 
-    [Test]
+		[Test]
 		public void CanResolveRootNode()
 		{
 			string assemblyName = null;
 			string clrNamespace = null;
 			string typeName = null;
 
-
 			XamlLoader.FallbackTypeResolver = (fallbackTypeInfos, type) =>
 			{
 				assemblyName = fallbackTypeInfos?[1].AssemblyName;

Xamarin.Forms.Xaml/ExpandMarkupsVisitor.cs

@@ -78,17 +78,28 @@ namespace Xamarin.Forms.Xaml
 			if (expression.StartsWith("{}", StringComparison.Ordinal))
 				return new ValueNode(expression.Substring(2), null);
 
-			if (expression[expression.Length - 1] != '}')
-				throw new Exception("Expression must end with '}'");
+			if (expression[expression.Length - 1] != '}') {
+				var ex = new XamlParseException("Expression must end with '}'", xmlLineInfo);
+				if (Context.ExceptionHandler != null) {
+					Context.ExceptionHandler(ex);
+					return null;
+				}
+				throw ex;
+			}
 
-			int len;
-			string match;
-			if (!MarkupExpressionParser.MatchMarkup(out match, expression, out len))
+			if (!MarkupExpressionParser.MatchMarkup(out var match, expression, out var len))
 				throw new Exception();
-			expression = expression.Substring(len).TrimStart();
-			if (expression.Length == 0)
-				throw new Exception("Expression did not end in '}'");
 
+			expression = expression.Substring(len).TrimStart();
+			if (expression.Length == 0) {
+				var ex = new XamlParseException("Expression did not end in '}'", xmlLineInfo);
+				if (Context.ExceptionHandler != null)
+				{
+					Context.ExceptionHandler(ex);
+					return null;
+				}
+				throw ex;
+			}
 			var serviceProvider = new XamlServiceProvider(node, Context);
 			serviceProvider.Add(typeof (IXmlNamespaceResolver), nsResolver);