Stub implementation of tls-api. All operations return an error.
Useful when you need an implementation of type like `TlsConnector`,
but you do not intend to use it.
E. g.
```
fn connect<C : tls_api::TlsConnector>(host: &str, use_tls: bool) { ... }
```
So if you want to use this function without TLS, you can call it with stub connector:
```
connect::<tls_api_stub::TlsConnector>("database", false);
```