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

interop/src/bin/client.rs

@@ -38,7 +38,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
             ClientTlsConfig::new()
                 .ca_certificate(ca)
                 .domain_name("foo.test.google.fr"),
-        );
+        )?;
     }
 
     let channel = endpoint.connect().await?;

interop/src/bin/server.rs

@@ -24,7 +24,7 @@ async fn main() -> std::result::Result<(), Box<dyn std::error::Error>> {
         let key = tokio::fs::read("interop/data/server1.key").await?;
         let identity = Identity::from_pem(cert, key);
 
-        builder = builder.tls_config(ServerTlsConfig::new().identity(identity));
+        builder = builder.tls_config(ServerTlsConfig::new().identity(identity))?;
     }
 
     let test_service = server::TestServiceServer::new(server::TestService::default());