Check for chain_head.dat on snapshot load

[heifner]

Currently we check that a shared_memory.bin doesn't exist when loading a snapshot. With the addition of chain_head.dat we should also check for that.

         auto shared_mem_path = chain_config->state_dir / "shared_memory.bin";
         EOS_ASSERT( !std::filesystem::is_regular_file(shared_mem_path),
                 plugin_config_exception,
                 "Snapshot can only be used to initialize an empty database." );