shard_reader: Continue after read_ahead error
shard_reader: Continue after read_ahead error

If read ahead failed, just issue a log warning
and proceed to close the reader.

Currently co_await will throw and the evictable reader
won't be closed.

This is seen occasionally in testing, e.g.
https://jenkins.scylladb.com/view/master/job/scylla-master/job/dtest-debug/1010/artifact/logs-all.debug.2/1640918573898_lwt_banking_load_test.py%3A%3ATestLWTBankingLoad%3A%3Atest_bank_with_nemesis/node2.log
```
ERROR 2021-12-31 02:40:56,160 [shard 0] mutation_reader - shard_reader::close(): failed to stop reader on shard 1: seastar::named_semaphore_timed_out (Semaphore timed out: _system_read_concurrency_sem)
```

Fixes #9865.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <20220102124636.2791544-1-23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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