trust-add: Catch correct exception when chown SSSD
    
    Commit 72fb4e6 introduced a regression. SSSD_USER.chown() raises
    ValueError instead of KeyError when SSSD user does not exist.
    
    Fixes: https://pagure.io/freeipa/issue/8516
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>