Upload processing fails when it encounters broken soft symlink

[ondracka]

See upload id an7fhANaSm-hJWTcneQ-8g at the beta server. Upload processing fails with:
Upload processing has errors: FileNotFoundError: [Errno 2] No such file or directory: '.volumes/fs/staging/a/an7fhANaSm-hJWTcneQ-8g/raw/LOBSTER/Ti/POTCAR'

The upload has the obvious problem that there are symlinks pointing to directory that was not included. However it should be probably handled more gracefully, i.e., treat the broken symlink like a standard file with size 0 and not abort the whole Upload processing at the first encounter.