bot: fix duplicate unregister KeyError and improve connection error logging

Silently ignore KeyError when unregistering an already-removed FD from
the poll loop (servers can queue multiple close events). Also include
the exception message when a server connection fails at startup.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nemubot/server/IRCLib.py

@@ -20,6 +20,7 @@ import threading
 
 import irc.bot
 import irc.client
+import irc.connection
 
 import nemubot.message as message
 from nemubot.server.threaded import ThreadedServer
@@ -278,7 +279,6 @@ class IRCLib(ThreadedServer):
 
         if self._ssl:
             import ssl as ssl_mod
-            import irc.connection
             ctx  = ssl_mod.create_default_context()
             host = self._host  # capture for closure
             connect_params["connect_factory"] = irc.connection.Factory(