Fix autosnapshot when the last slot in an epoch doesn't contain a block

[buffalu]

• autosnapshot errors out if the last slot in the epoch doesn't contain a block from this PR: create-snapshot check if snapshot slot exists solana-labs/solana#27153

Fix:

• starting at the last slot in the epoch, loop backwards until there's a block that returns a result and take a snapshot there.

[esemeniuc]

does this check not get executed due to set -eux on line 9?

[esemeniuc]

eg

#!/bin/bash

set -exu
function foo {
  rm aldskjdjkldlkjadskjldkjlskjlkjdlslkjda
  echo $?
}

foo

echo 'finished'

[esemeniuc]

➜  ~ bash demo.sh
+ foo
+ rm aldskjdjkldlkjadskjldkjlskjlkjdlslkjda
rm: cannot remove 'aldskjdjkldlkjadskjldkjlskjlkjdlslkjda': No such file or directory
➜  ~ echo $?
1

the echo finished never runs

[buffalu]

yeah im not totally sure why it doesn't work, there's a comment above it.