blockchain remote-fetch and pull crash on locally nonexistent blockchain

[mzabaluev]

This command fails when the blockchain is not initialized with new or has been destroyed:

cardano-cli blockchain remote-fetch mainnet hermes

The backtrace excerpt:

   9: <core::option::Option<T>>::unwrap
             at libcore/macros.rs:20
  10: cardano_cli::blockchain::Blockchain::load
             at src/blockchain/mod.rs:97
  11: cardano_cli::blockchain::commands::remote_fetch
             at src/blockchain/commands.rs:165
  12: cardano_cli::subcommand_blockchain
             at src/main.rs:300
  13: cardano_cli::main
             at src/main.rs:77