EMSUSD-489 fix reparenting crash

[pierrebai-adsk]

There were multiple problems that triggered each others to cause a crash when using relative sub-layers. The problems were:

• The relative sub-layer was not found.
• This cause the code that applies the reparenting to all layers to do no work.
• This would cause the prim to vanish.
• This would cause the transform command to crash because it did not check that the scene item was valid.

The corrections done are:

• Fix the getAllSubLayers helper function to use FindRelativeToLayer to find sub-layers, in case they are relative to their parent. Otherwise it would fail to find sub-layers.
• Modify the applyToAllPrimSpecs, applyToAllLayersWithOpinions and applyToSomeLayersWithOpinions helper functions to return the number of layers that were affected.
• This allows the caller to detect if no action was taken at all, indicating that an expected action failed.
• Make the UsdSetMatrix4dUndoableCmd UFE command not crash when the scene item is not found.
• Make the insert-child UFE command detect if the move to the new parent did no work at all and report an error instead of marching on and deleting the old location.
• Otherwise, the prim would not be at the new location and would be deleted from its old location, so it owuld vanish.

Add a unit test that try to reparent a prim in a relative sub-layer. It failed without the fixes, now it works.

[seando-adsk]

Great work. Thanks for adding the error checking and unit test.