Print error message before crash

If our IOPS methods raise an exception, the iops internals can't deal
with that and will deref a null pointer. Let's print the exception and
then abort, rather than segfault blindly

Change-Id: Icdb0f0b5d3513482223269e30d910329c3b2e603
Reviewed-on: http://review.couchbase.org/30811
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>