software/proftpd/test: relax assertion on TestBan

We don't care about exact exception message, but we only want to test
that the client can't connect anymore once banned. Relaxing this
assertion should stabilize previously flaky
'TestBan.test_client_are_banned_after_5_wrong_passwords'.

See nexedi/slapos@97f4a5fb (comment 150049)
and nexedi/slapos!1305 (comment 175255)
for additional context.