invite: handle expected possible exception [skip ci]

Avoids "Unknown error" output to IRC and exception traceback log dump.

Backport of #2151 for 7.1.x maintenance branch. CI skipped as the change
does not touch testable code.

sopel/modules/invite.py

@@ -77,4 +77,7 @@ def invite(bot, trigger):
                 .format(bot.config.core.help_prefix))
         channel = trigger.sender
 
-    invite_handler(bot, trigger.nick, user, channel)
+    try:
+        invite_handler(bot, trigger.nick, user, channel)
+    except ValueError as err:
+        bot.reply('%s' % err)