Reduce RPC response error logging level

These can happen pretty often for missing mempool txids and do not
warrant warn-level logging. Unexpected RPC errors will bubble up and
be handled appropriately.

src/daemon.rs

@@ -250,7 +250,7 @@ impl Connection {
  Ok(if status == "HTTP/1.1 200 OK" {
  contents
  } else if status == "HTTP/1.1 500 Internal Server Error" {
- warn!("HTTP status: {}", status);
+ debug!("RPC HTTP 500 error: {}", contents);
  contents // the contents should have a JSONRPC error field
  } else {
  bail!(