`GLTFDocument.write_to_filesystem` squashes file IO errors into single `FAILED` error on Linux

### Tested versions

- Reproducible in Godot v4.6.2.stable.mono (001aa128b)

### System information

Godot v4.6.2.stable.mono (001aa128b) - Arch Linux #1 ZEN SMP PREEMPT_DYNAMIC Fri, 01 May 2026 16:30:13 +0000 on Wayland - Wayland display driver, Multi-window, 2 monitors - OpenGL 3 (Compatibility) - NVIDIA GeForce RTX 4060 Ti (nvidia; 595.71.05) - AMD Ryzen 7 5700X3D 8-Core Processor (16 threads) - 78.44 GiB memory

### Issue description

`GLTFDocument.write_to_filesystem` returns a generic `FAILED` error instead of the actual IO error that prevented writing. The farthest I've managed to track it down is to

https://github.com/godotengine/godot/blob/e880d6bbfb74479376a0cba9b106ac0c39b4cfe4/modules/gltf/gltf_document.cpp#L6675

The context of this line is below, but I'm not sure where the `FAILED` error code is actually coming from, since `_encode_bundle_bins` doesn't seem to return a `FAILED` ever, nor does `FileAccess::open` which it calls

https://github.com/godotengine/godot/blob/e880d6bbfb74479376a0cba9b106ac0c39b4cfe4/modules/gltf/gltf_document.cpp#L6675-L6682

### Steps to reproduce

Create a `gltf` file owned by root in your home directory
`sudo touch ~/testfile.gltf`

Then run the below in a copy of Godot running as a normal user
```gdscript
var doc := GLTFDocument.new()
var state := GLTFState.new()
var err := doc.write_to_filesystem(state, "~/testfile.gltf")
print("Error was %s" % error_string(err)) # Should print 'Failed'
```

### Minimal reproduction project (MRP)

N/A

Start in modules/gltf/gltf_document.cpp around lines 6675-6682, then trace write_to_filesystem into _encode_bundle_bins and FileAccess::open. Reproduce the root-owned-file case and compare the returned error with the underlying file I/O failure. Done means write_to_filesystem exposes the actual I/O error instead of a generic FAILED result.

Written by the indexing model from the issue text.