More clarity for error when spawnables reference level actors

More clarity for error when spawnables reference level actors

I believe this error is expected as spawnables shouldn't reference level actors, but if not, we can convert this to a bug to resolve this limitation.

Can't save ../../../../../../Unreal Projects/SequencerTestBed/Content/Test_LookAtSpawnableCam/Test_LookAtSpawnableCam_SQ.uasset: Graph is linked to object(s) in external map.
External Object(s):
Floor_1
  
Try to find the chain of references to that object (may take some time)?

1. Create a sequence and add a spawnable Cine Camera track
2. Select the spawnable Cine Camera
3. Enable Tracking and choose a level actor to track
4. Attempt to save the sequence

Result: Error in description
Expected: Error with a clearer description. Some users having trouble figuring it out