fix(transport): Propagate errors in tls_config instead of unwrap/panic (#385)

* Propagate errors in tls_config instead of unwrap

Ran into `tls_connector` failing and causing our app to panic and shutdown as it seems there wasn't any way to avoid panicking in `tls_config`.

So after talking to @LucioFranco briefly `tls_config` now returns a `Result` instead and propagates errors to the caller, where they can be handled.

* Fix compile warning when tls feature is disabled

examples/src/gcp/client.rs

@@ -32,7 +32,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         .domain_name("pubsub.googleapis.com");
 
     let channel = Channel::from_static(ENDPOINT)
-        .tls_config(tls_config)
+        .tls_config(tls_config)?
         .connect()
         .await?;