Provide more context when scene fails to load

[KoBeWi]

Partially addresses #84981

Before this PR:

scene/resources/packed_scene.cpp:141 - Condition "nc == 0" is true. Returning: nullptr
scene/resources/packed_scene.cpp:222 - Parameter "node" is null.
scene/resources/packed_scene.cpp:222 - Parameter "node" is null.
editor/editor_data.cpp:626 - Index p_idx = 1 is out of bounds (edited_scene.size() = 1).

After this PR:

C:\godot_source\scene/resources/packed_scene.cpp:141 - Condition "nc == 0" is true. Returning: nullptr
Failed to load scene dependency: "res://Content/Cards/Spells/spell_container.tscn". Make sure the required scene is valid.
Failed to load scene dependency: "res://Systems/Characters/Player/player.tscn". Make sure the required scene is valid.
C:\godot_source\editor/editor_data.cpp:626 - Index p_idx = 1 is out of bounds (edited_scene.size() = 1).

[akien-mga]

Since it seems to turn up frequently in user logs, it could be good to also give a clearer message to the nc == 0 error, even if it's not directly actionable to users.

[KoBeWi]

Failed to instantiate scene state of "res://Content/Cards/Spells/spell_container.tscn", node count is 0. Make sure the PackedScene resource is valid.
Failed to load scene dependency: "res://Content/Cards/Spells/spell_container.tscn". Make sure the required scene is valid.
Failed to load scene dependency: "res://Systems/Characters/Player/player.tscn". Make sure the required scene is valid.

Seems like scene state path is available and the error comes from the first failed dependency.

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.1.4.