fix: resolve issue retrieving open error

[patoui]

Description

This pull request fixes an issue when the plugin is unable to open a file, when the aseprite file generated bad data.

Current behaviour:

  res://addons/AsepriteWizard/aseprite/file_exporter.gd:138 - Attempt to call function 'get_open_error' in base 'null instance' on a null instance.
  res://addons/AsepriteWizard/creators/animation_player/animation_creator.gd:18 - Invalid get index 'is_ok' (on base: 'Dictionary').
  res://addons/AsepriteWizard/creators/animation_player/animation_creator.gd:10 - Invalid type in function 'get_error_message' in base 'GDScript'. Cannot convert argument 1 from Nil to int.

Expected behaviour:

aseprite generated bad data file

[viniciusgerevini]

Thank you!