ledger-tool: Use error handling in blockstore command code (solana-la…

…bs#35157)

There are lots of operations that could fail, including lots of the
Blockstore calls. The old code matched on Ok(_) or did unwrap()'s
which clutter the code and increase indentation.

This change wraps the entire command in a function that returns a
Result. The wrapper then does a single unwrap_or_else() and prints
any error message. Everywhere else is now free to use the ? operator