Add error message for lacking consumer startup

Instead of relying on the user to interpret the existing debug logs and
badmatch error, send an explicit error message when no consumer could be
found that we can use in the shard connection. See #524.

lib/nostrum/shard/session.ex

@@ -343,7 +343,14 @@ defmodule Nostrum.Shard.Session do
       {^reference, :join, _group, _who} ->
         :ok
     after
-      timeout -> :timeout
+      timeout ->
+        Logger.error(
+          "No consumers were running nor did any start up in time " <>
+            "for shard session startup. Is a consumer started as " <>
+            "part of your supervision tree?"
+        )
+
+        :timeout
     end
   end
 end