dtests: fix pytest.raises argument names

I've been through a couple of dtest results lately and notices some interpreter errors regarding how we call pytest.raises. The reference is pretty clear on what would be the correct arguments, but still want to make sure we're not working on different pytest versions.
Michael Kjellman can you quickly check the following inconsistencies and look at my patch (msg->message, matches->match)?

git show 49b2dda4 |egrep 'raises.*, m'