[Audio] Restart Lavalink after unexpected shutdown (#3034)

* fix(audio): lavalink not getting restarted after unexpected shutdown

* chore(changelog): add towncrier entry

redbot/cogs/audio/manager.py

@@ -59,7 +59,7 @@ class ServerManager:
         if self._proc is not None:
             if self._proc.returncode is None:
                 raise RuntimeError("Internal Lavalink server is already running")
-            else:
+            elif self._shutdown:
                 raise RuntimeError("Server manager has already been used - create another one")
 
         await self.maybe_download_jar()