Don't throw in LogicalNotNode.SetValue.

Instead return false to indicate the write failed.

src/Markup/Perspex.Markup/Data/LogicalNotNode.cs

@@ -11,7 +11,7 @@ namespace Perspex.Markup.Data
     {
         public override bool SetValue(object value)
         {
-            throw new NotSupportedException("Cannot set a negated binding.");
+            return false;
         }
 
         public override IDisposable Subscribe(IObserver<object> observer)

tests/Perspex.Markup.UnitTests/Data/ExpressionObserverTests_Negation.cs

@@ -81,12 +81,12 @@ namespace Perspex.Markup.UnitTests.Data
         }
 
         [Fact]
-        public void SetValue_Should_Throw()
+        public void SetValue_Should_Return_False()
         {
             var data = new { Foo = "foo" };
             var target = new ExpressionObserver(data, "!Foo");
 
-            Assert.Throws<NotSupportedException>(() => target.SetValue("bar"));
+            Assert.False(target.SetValue("bar"));
         }
     }
 }