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/ILive.cs

@@ -31,6 +31,11 @@ namespace osu.Game.Database
         /// <param name="perform">The action to perform.</param>
         void PerformWrite(Action<T> perform);
 
+        /// <summary>
+        /// Whether this instance is tracking data which is managed by the database backing.
+        /// </summary>
+        bool IsManaged { get; }
+
         /// <summary>
         /// Resolve the value of this instance on the current thread's context.
         /// </summary>