[coro_rpc][breakchange] enhance error handling

[poor-circle]

Why

What is changing

1. use enum class coro_rpc::errc : uint8_t instead of std::errc for cross-platform.
2. add support for user set error by coro_rpc::context.
3. add support for user-defined framework error code.(It should between 100-255)

Example

void hello(coro_rpc::context<void> ctx) {
  ctx.response_error(coro_rpc::errc::io_error,"This is io error.");
}