samba: Ignore non-existent users who are in freedombox-share group

Fixes error 500 when trying to open Samba app page when at least one of the users in freedombox-share group doesn't exist.

Tests performed in both stable and testing containers:

• Installed Samba app.
• Installed Deluge app (Plinth install/setup fails but the Debian package itself was installed).
• Did apt remove --purge deluged from command line.
• Checked that the debian-deluged user doesn't exist (getent passwd) and the user is in the freedombox-share group (getent group).
• Checked that the Samba app page opens without errors.
• Checked that all the Samba tests pass.

Fixes #2411 (closed).

Signed-off-by: Veiko Aasa veiko17@disroot.org