Implement std::error::Error for our custom errors

For allowing a more ergonomic use and better integration on the
ecosystem, this adds the `std::error::Error` `impl` for our custom
errors.

We intent to drop this hand made code once `derive_more` finishes the
addition of the Error derive support[1]. Until that is available, we
need to live with that.

1. https://github.com/JelteF/derive_more/issues/92

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>

actix-http/src/ws/mod.rs

@@ -58,6 +58,8 @@ pub enum ProtocolError {
     Io(io::Error),
 }
 
+impl std::error::Error for ProtocolError {}
+
 impl ResponseError for ProtocolError {}
 
 /// Websocket handshake errors