dsdb python tests: convert 'except X, (tuple)' to 'except X as e'

dsdb python tests: convert 'except X, (tuple)' to 'except X as e'

In addition to converting the except line another line is also added
for each except to extract the tuple contents.

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>