Make failing to create a command binding suck less

Author: anaisbetts

ReactiveUI.Platforms/Xaml/CreatesCommandBinding.cs

@@ -50,7 +50,12 @@ public IDisposable BindCommandToObject(ICommand command, object target, IObserva
                 .Select(x => new { EventInfo = type.GetRuntimeEvent(x.Item1), Args = x.Item2 })
                 .FirstOrDefault(x => x.EventInfo != null);
 
-            if (eventInfo == null) return null;
+            if (eventInfo == null) {
+                throw new Exception(
+                    String.Format(
+                        "Couldn't find a default event to bind to on {0}, specify an event expicitly", 
+                        target.GetType().FullName));
+            }
 
             var mi = GetType().GetRuntimeMethods().First(x => x.Name == "BindCommandToObject" && x.IsGenericMethod);
             mi = mi.MakeGenericMethod(eventInfo.Args);
@@ -128,4 +133,4 @@ public IDisposable BindCommandToObject<TEventArgs>(ICommand command, object targ
             return null;
         }
     }
-}
+}