ipa-backup: better error message if ENOSPC
    
    When the destination directory cannot store the complete backup
    ipa-backup fails but does not explain why.
    This commit adds error-checking to db2ldif(), db2bak() and
    finalize_backup() and enhances the error message.
    
    Fixes: https://pagure.io/freeipa/issue/7647
    Signed-off-by: François Cami <fcami@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Tibor Dudlák <tdudlak@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Thomas Woerner <twoerner@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>