Improve error messages for invalid resources imported in the editor

[Calinou]

• Follow-up to Improve error messages in ResourceLoader #106720 (can be merged independently).

The following situations will now print bespoke error messages:

• Importing an empty resource (0 bytes). This is generally a corrupted file (written during a crash or power outage) where the OS leaves an empty file.
• Importing a Git LFS pointer (which always starts with the same first line) instead of the actual file data. Running git lfs pull replaces this pointer with the actual file data.

Testing project: test_invalid_files.zip

Preview

WARNING: Not a PNG file
     at: check_error (./drivers/png/png_driver_common.cpp:55)
ERROR: Condition "!success" is true. Returning: ERR_FILE_CORRUPT
   at: png_to_image (./drivers/png/png_driver_common.cpp:68)
ERROR: Error loading image: 'res://actually_a_jpg.png'.
   at: load_image (./core/io/image_loader.cpp:100)
ERROR: Error importing 'res://actually_a_jpg.png'.
   at: _reimport_file (./editor/editor_file_system.cpp:3024)
ERROR: Resource file is a Git LFS pointer (run `git lfs pull` to fetch it): res://lfs_pointer.png
   at: _reimport_file (./editor/editor_file_system.cpp:2767)
ERROR: Resource file is corrupt (empty size): res://empty.png
   at: _reimport_file (./editor/editor_file_system.cpp:2751)

[Calinou]

There's a similar check in place in the OBJ importer, although it's designed to check if the file is binary instead of being text: https://github.com/godotengine/godot/blob/master/editor/import/3d/resource_importer_obj.cpp#L204

It's likely possible to write this in a more robust/efficient manner, but it probably works well enough as-is.

[bruvzg]

It's a simple ASCII string, so it might be better to just check it in the loop instead of using get_line(), which would still print errors if file start with vers` and have invalid UTF-8 sequences.

	const char *sample = "version https://git-lfs";
	bool match = true;
	for (int i = 0; sample[i] != 0; i++) {
		if (f->eof_reached() || sample[i] != f->get_8()) {
			match = false;
			break;
		}
	}
	ERR_FAIL_COND_V_MSG(match, ERR_FILE_UNRECOGNIZED,
				vformat("Resource file is a Git LFS pointer (run `git lfs pull` to fetch it): %s", p_file));