Avoid attempting to fetch a non-managed RealmLive instance from the realm backing

For compatibility reasons, we quite often convert completely unmanaged
instances to `ILive`s so they fit the required parameters of a property
or method call. This ensures such cases will not cause any issues when
trying to interact with the underlying data.

Originally I had this allowing write operations, but that seems a bit
unsafe (when performing a write one would assume that the underlying
data is being persisted, whereas in this case it is not). We can change
this if the requirements change in the future, but I think throwing is
the safest bet for now.

osu.Game/Database/EntityFrameworkLive.cs

@@ -9,6 +9,7 @@ namespace osu.Game.Database
     {
         public EntityFrameworkLive(T item)
         {
+            IsManaged = true; // no way to really know.
             Value = item;
         }
 
@@ -29,6 +30,8 @@ namespace osu.Game.Database
             perform(Value);
         }
 
+        public bool IsManaged { get; }
+
         public T Value { get; }
     }
 }