Improve unimplemented message (yewstack#716)

* Improve unimplemented message

When calling Bridged::bridge, it is possible to get a confusing
unimplemented error. This PR adds some explanation and clarity around
the error.

* Update link to point to latest

* cargo fmt

src/agent.rs

@@ -205,7 +205,11 @@ where
 pub trait Discoverer {
     /// Spawns an agent and returns `Bridge` implementation.
     fn spawn_or_join<AGN: Agent>(_callback: Option<Callback<AGN::Output>>) -> Box<dyn Bridge<AGN>> {
-        unimplemented!();
+        unimplemented!(
+            "The Reach type that you tried to use with this Agent does not have
+Discoverer properly implemented for it yet. Please see
+https://docs.rs/yew/latest/yew/agent/ for other Reach options."
+        );
     }
 }