Don't show warnings in log when scanning for files in non-existent dirs

[olanti-p]

Summary

SUMMARY: Infrastructure "Don't show warnings in log when scanning for files in non-existent dirs"

Purpose of change

There are warnings about missing directories in debug.log when game tries to load data from nonexistent user dirs:

15:11:02.060 WARNING : opendir [./user_dir/sound/] failed with "No such file or directory".
15:11:02.060 WARNING : opendir [./user_dir/gfx/] failed with "No such file or directory".
15:11:02.060 INFO : Number of render drivers on your system: 3
15:11:02.060 INFO : Render driver: 0/opengl
15:11:02.060 INFO : Render driver: 1/opengles2
15:11:02.060 INFO : Render driver: 2/software
15:11:02.073 WARNING : opendir [./user_dir/mods/] failed with "No such file or directory".

These are entirely benign, but the word "failed" seems to be an attention grabber whenever people try to figure out why their game has broken.

Describe the solution

If folder does not exist, just return an empty list.