MariaRocks port: Make SQL layer allow errors in start_consistent_snapshot().
MariaRocks port: Make SQL layer allow errors in start_consistent_snapshot().

rocksdb.cons_snapshot_read_committed test used to crash.
This happened, because
- MyRocks produces an error when one attempts to do a START TRANSACTION
  WITH CONSISTENT SNAPSHOT with @@tx_isolation=REPEATABLE_READ
- MariaDB's SQL layer didn't assume that errors are possible inside
  hton->start_constistent_snapshot() call.

natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2