exp/lighthorizon: single watch command stops after not finding latest ledger tx meta

[sreuland]

What version are you using?

latest build of lighthorizon

What did you do?

./single ... -watch=TRUE ...

What did you expect to see?

the watch state continues to ingest latest new ledgers

What did you see instead?

the single command exited early with logs:

time="2022-09-09T22:27:37Z" level=warning msg="http: Bad" method=GET status="404 Not Found" url="https://s3.amazonaws.com/horizon-ledgermeta-prodnet-test/ledgers/42598655"
time="2022-09-09T22:27:37.161Z" level=error msg="error getting ledger 42598655: NoSuchKey: The specified key does not exist.\n\tstatus code: 404, request id: WF365TZ22NX0V1SB, host id: 9tO0rASAQkb8shbLSi8LGYxcMtNbG/bS0rsH/HnxatAR2qn3l68m1n5F23S5Rq5xRL6R5dmIDRQ=" pid=7
panic: awaiting next ledger failed: NoSuchKey: The specified key does not exist.
	status code: 404, request id: WF365TZ22NX0V1SB, host id: 9tO0rASAQkb8shbLSi8LGYxcMtNbG/bS0rsH/HnxatAR2qn3l68m1n5F23S5Rq5xRL6R5dmIDRQ=

goroutine 1 [running]:
main.main()
	/go/src/github.com/stellar/go/exp/lighthorizon/index/cmd/single/main.go:49 +0x367