Do not suppress errors on writes outside transactions.

Do not suppress errors on writes outside transactions.
Mike Wray made this change because xend's callbacks were failing
with EROFS, because a transaction was going on.  However, the
solution is to either block callbacks during transactions, or use
a different connection to the store.
We fail ex-transaction activity because it's not clear what the right
behaviour is: it allows deadlock if two people do it, and it implied
your transaction isn't rooted in the right place.

This hack caused the testsuite to fail (obviously), so as a result the
testsuite wasn't run very often.  This caused it to bitrot.
Signed-off-by: Rusty Russel <rusty@rustcorp.com.au>
Signed-off-by: Christian Limpach <Christian.Limpach@cl.cam.ac.uk>