Bag creation is failing on hidden files

If the app encounters hidden files it will error out with this message. This happens even if the `--no-system-files` flag is used.

```
Traceback (most recent call last):
File "/usr/local/bin/grabbags", line 33, in
sys.exit(load_entry_point('grabbags==0.0.2', 'console_scripts', 'grabbags')())
File "/usr/local/Cellar/grabbags/0.0.2/libexec/lib/python3.9/site-packages/grabbags/grabbags.py", line 372, in main
runner(args)
File "/usr/local/Cellar/grabbags/0.0.2/libexec/lib/python3.9/site-packages/grabbags/grabbags.py", line 295, in run
make_bag(bag_dir, args)
File "/usr/local/Cellar/grabbags/0.0.2/libexec/lib/python3.9/site-packages/grabbags/grabbags.py", line 259, in make_bag
grabbags.utils.remove_system_files(root=bag_dir.path)
File "/usr/local/Cellar/grabbags/0.0.2/libexec/lib/python3.9/site-packages/grabbags/utils.py", line 79, in remove_system_files
os.remove(full_path)
FileNotFoundError: [Errno 2] No such file or directory: '/Volumes/SymplyUltra/Testing/TBC Presentation1/20191203_SDI/._.DS_Store'
```