blender/import: correct exit code on Python exception

[kroketio]

When running Blender direct import, it executes some Python but does not check if it succeeded.

https://github.com/godotengine/godot/blob/607b230ffe120b2757c56bd3d52a7a0d4e502cfe/modules/gltf/editor/editor_import_blend_runner.cpp#L84-L93

Which goes to start_blender(...) and exitcode is checked to determine failure.

https://github.com/godotengine/godot/blob/607b230ffe120b2757c56bd3d52a7a0d4e502cfe/modules/gltf/editor/editor_import_blend_runner.cpp#L161-L175

However, Blender does not return exitcode 1 when the Python script fails: blender/issues/82494

There is --python-exit-code which returns the exitcode in the given value whenever the script fails:

$ blender --help
--python-exit-code <code>
        Set the exit-code in [0..255] to exit if a Python exception is raised
        (only for scripts executed from the command line), zero disables.

This PR adds the flag, as well as print the stderr of the Blender process for more clarity.