--fail-immediately in filter-subunit

[tridge at samba.org]

Hi Jelmer,

If you have time, can you have a look at my subunit-fail-immediately
branch? I'm trying to make filter-subunit fail on the first test
failure when we use the autobuild system, so others who are behind
that committer don't have to wait so long.

I've added the basic logic, but the exception I raise in FilterOps()
doesn't kill the whole test - other processes keep trying to write and
get lots of pipe errors. I'm hoping you can work out why.

Thanks!

Cheers, Tridge