Make sure bound property is registered.

Perspex.Base/PerspexObject.cs

@@ -574,6 +574,14 @@ namespace Perspex
             PriorityValue v;
             IDescription description = source as IDescription;
 
+            if (!this.IsRegistered(property))
+            {
+                throw new InvalidOperationException(string.Format(
+                    "Property '{0}' not registered on '{1}'",
+                    property.Name,
+                    this.GetType()));
+            }
+
             if (!this.values.TryGetValue(property, out v))
             {
                 v = this.CreatePriorityValue(property);

Tests/Perspex.Base.UnitTests/PerspexObjectTests.cs

@@ -355,6 +355,17 @@ namespace Perspex.Base.UnitTests
             Assert.Equal("initial", target.GetValue(Class1.FooProperty));
         }
 
+        [Fact]
+        public void Bind_Throws_Exception_For_Unregistered_Property()
+        {
+            Class1 target = new Class1();
+
+            Assert.Throws<InvalidOperationException>(() =>
+            {
+                target.Bind(Class2.BarProperty, Observable.Return("foo"));
+            });
+        }
+
         [Fact]
         public void Bind_Sets_Subsequent_Value()
         {