Fix handling of missing bin file in gltf separate

[Jordyfel]

Fix #85177

The missing .bin file case was not handled very well, because the error check after attempting to load it checked the wrong variable, so the function returned OK and errors popped up elsewhere.

Since it's very possible for the user to miss moving the .bin when moving gltf separate in the editor filesystem (because it isn't visible), adding another check to more explicitly complain that the file is missing.

Errors before:

  Can't open file from path 'res://untitled.bin'.
  modules/gltf/gltf_document.cpp:1323 - Condition "(int)(offset + buffer_end) > buffer.size()" is true. Returning: ERR_PARSE_ERROR
  modules/gltf/gltf_document.cpp:1323 - Condition "(int)(offset + buffer_end) > buffer.size()" is true. Returning: ERR_PARSE_ERROR
  modules/gltf/gltf_document.cpp:2868 - Condition "vertex_num <= 0" is true. Returning: ERR_INVALID_DECLARATION
  modules/gltf/gltf_document.cpp:7251 - Condition "err != OK" is true. Returning: ERR_PARSE_ERROR
  modules/gltf/gltf_document.cpp:6895 - Condition "err != OK" is true. Returning: err
  modules/gltf/gltf_document.cpp:7426 - Condition "err != OK" is true. Returning: err
  Error importing 'res://untitled.gltf'.

Errors after:

  glTF: Binary file not found: res://untitled.bin
  modules\gltf\gltf_document.cpp:7207 - Condition "err != OK" is true. Returning: ERR_PARSE_ERROR
  modules\gltf\gltf_document.cpp:6896 - Condition "err != OK" is true. Returning: err
  modules\gltf\gltf_document.cpp:7427 - Condition "err != OK" is true. Returning: err
  Error importing 'res://untitled.gltf'.

[akien-mga]

Thanks!