nbd: Add some error case testing to iotests 223

nbd: Add some error case testing to iotests 223

Testing success paths is important, but it's also nice to highlight
expected failure handling, to show that we don't crash, and so that
upcoming tests that change behavior can demonstrate the resulting
effects on error paths.

Add the following errors:
Attempting to export without a running server
Attempting to start a second server
Attempting to export a bad node name
Attempting to export a name that is already exported
Attempting to export an enabled bitmap
Attempting to remove an already removed export
Attempting to quit server a second time

All of these properly complain except for a second server-stop,
which will be fixed next.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190111194720.15671-2-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>