fix (DatabaseLogEvent): add stream_exception as a new database error event
fix (DatabaseLogEvent): add stream_exception as a new database error event

Currently stream_exception occurred in Longevity-200gb-48h of 4.1.rc2,
but the error wasn't filtered out, the problem cluster was killed.

Error example:
scylla: [shard 0] range_streamer - Bootstrap with 10.0.45.208 for \
  keyspace=keyspace1 failed, took 0.049 seconds: \
  streaming::stream_exception (Stream failed)
scylla: [shard 0] boot_strapper - Error during bootstrap: \
  streaming::stream_exception (Stream failed)

Signed-off-by: Amos Kong <eb01d8f828a6c9a20be4534e72e049aaf41503df@scylladb.com>

scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests