streaming: stream_session: do not escape curly braces in format strings
streaming: stream_session: do not escape curly braces in format strings

Those turn into '{}' in the formatted strings and trigger
a logger error in the following sstlog.warn(err.c_str())
call.

Fixes #8436

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <20210408173048.124417-1-23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla