gossip: Handle timeout error in gossiper::do_shadow_round
gossip: Handle timeout error in gossiper::do_shadow_round

Currently, the rpc timeout error for the GOSSIP_GET_ENDPOINT_STATES verb
is not handled in gossiper::do_shadow_round. If the
GOSSIP_GET_ENDPOINT_STATES rpc call to any of the remote nodes goes
timeout, gossiper::do_shadow_round will throw an exception and fail the
whole boot up process.

It is fine that some of the remote nodes timeout in shadow round. It is
not a must to talk to all nodes.

This patch fixes an issue we saw recently in our sct tests:

```
INFO    | scylla[1579]: [shard 0] init - Shutting down gossiping
INFO    | scylla[1579]: [shard 0] gossip - gossip is already stopped
INFO    | scylla[1579]: [shard 0] init - Shutting down gossiping was successful
...

ERR     | scylla[1579]: [shard 0] init - Startup failed: seastar::rpc::timeout_error (rpc call timed out)
```

Fixes #8187

Closes #8213

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