When a transaction fails in parallel replication, it should signal the error
to any following transactions doing wait_for_prior_commit() on it. But the
code for this was incorrect, and would not correctly remember a prior error
when sending the signal. This caused corruption when slave stopped due to an
error.

Fix by remembering the error code when we first get an error, and passing the
saved error code to wakeup_subsequent_commits().

Thanks to nanyi607rao who reported this bug on
maria-developers@lists.launchpad.net and analysed the root cause.