28344: storage: don't return status error on rollbacks r=andreimatei a=andreimatei

Before this patch, EndTransaction(commit=false) would return
TransactionStatusError if the txn record was not found (just like a
commit). This patch makes it return success (and an unmodified txn proto
in the response).
The motivation is that the client is routinely sending rollbacks in
situations where it's unclear whether the txn record has previously been
written successfully. We currently mostly swallow those errors on the
client, but that code is ugly and incomplete - the client sometimes turn
commits into rollbacks if all writes have been performed at an old
epoch. In that case we have a problem because, if the rollback encouters
the error, we might not have the responses for other requests in its
batch. So, I will take the client out of the business of massaging
these errors.

Release note: none

Co-authored-by: Andrei Matei <andrei@cockroachlabs.com>