Panic when the number of replicas is bigger than `max_wal_senders`

When you try to add one replica beyond `max_wal_senders` it panics on boot.
```sh
P2023-03-20T22:09:14Z app[21781770c63089] den [info]Provisioning standby
2023-03-20T22:09:15Z app[21781770c63089] den [info]repmgr -h fdaa:0:c688:a7b:d5a6:6646:f15a:2 -p 5433 -d repmgr -U repmgr -f /data/repmgr.conf standby clone -c -F
2023-03-20T22:09:22Z app[21781770c63089] den [info]panic: failed to clone primary: failed to clone primary: exit status 1
2023-03-20T22:09:22Z app[21781770c63089] den [info]goroutine 1 [running]:
2023-03-20T22:09:22Z app[21781770c63089] den [info]main.panicHandler({0x9a0c40?, 0xc0004303e0})
2023-03-20T22:09:22Z app[21781770c63089] den [info] /go/src/github.com/fly-examples/fly-postgres/cmd/start/main.go:100 +0x55
2023-03-20T22:09:22Z app[21781770c63089] den [info]main.main()
2023-03-20T22:09:22Z app[21781770c63089] den [info] /go/src/github.com/fly-examples/fly-postgres/cmd/start/main.go:34 +0xadd
```

We should handle this gracefully by logging an error or even return one to the user. Perhaps we should even automatically check the `max_wal_senders` setting and optionally update it before adding the new replica.