Prevent errors on broken SFTP config

Steps to reproduce:

1. Create a valid SFTP config with an invalid root (`~` and `$HOME` were tested)
2. Attempt to view the main Files page
3. Notice 'Unknown error' and errors in owncloud.log
4. Retry with fix, no errors

EDIT: 'Unknown error' may be a red herring, it just happened to me with a completely valid SFTP config (which works for browsing files!)