Handling item-not-found case in RPC methods is difficult

[casey]

For some RPC methods, like get_block_hash, get_block, and get_raw_transaction, it's often nice to handle the case where the requested item isn't found specially. I was able to do this, code reproduced below, but it's rather messy. It would be nice if such functions returned Option<T> instead of T, to avoid needing to write special-case code.

fn block_at_height(client: &Client, height: u64) -> Result<Option<Block>> {
  match client.get_block_hash(height) {
    Ok(hash) => Ok(Some(client.get_block(&hash)?)),
    Err(bitcoincore_rpc::Error::JsonRpc(bitcoincore_rpc::jsonrpc::error::Error::Rpc(
      bitcoincore_rpc::jsonrpc::error::RpcError { code: -8, .. },
    ))) => Ok(None),
    Err(bitcoincore_rpc::Error::JsonRpc(bitcoincore_rpc::jsonrpc::error::Error::Rpc(
      bitcoincore_rpc::jsonrpc::error::RpcError { message, .. },
    )))
      if message == "Block not found" =>
    {
      Ok(None)
    }
    Err(err) => Err(err.into()),
  }
}

pub(crate) fn block_with_hash(&self, hash: sha256d::Hash) -> Result<Option<Block>> {
  match self.client.get_block(&BlockHash::from_hash(hash)) {
    Ok(block) => Ok(Some(block)),
    Err(bitcoincore_rpc::Error::JsonRpc(bitcoincore_rpc::jsonrpc::error::Error::Rpc(
      bitcoincore_rpc::jsonrpc::error::RpcError { code: -8, .. },
    ))) => Ok(None),
    Err(bitcoincore_rpc::Error::JsonRpc(bitcoincore_rpc::jsonrpc::error::Error::Rpc(
      bitcoincore_rpc::jsonrpc::error::RpcError { message, .. },
    )))
      if message == "Block not found" =>
    {
      Ok(None)
    }
    Err(err) => Err(err.into()),
  }
}

pub(crate) fn transaction(&self, txid: Txid) -> Result<Option<Transaction>> {
  match self.client.get_raw_transaction(&txid, None) {
    Ok(transaction) => Ok(Some(transaction)),
    Err(bitcoincore_rpc::Error::JsonRpc(bitcoincore_rpc::jsonrpc::error::Error::Rpc(
      bitcoincore_rpc::jsonrpc::error::RpcError { code: -8, .. },
    ))) => Ok(None),
    Err(err) => Err(err.into()),
  }
}