tell: example tests for the error cases

Can't put nickname commands here because the pytest plugin doesn't
handle them. That'll be a future patch...

sopel/modules/tell.py

@@ -169,7 +169,9 @@ def _format_safe_lstrip(text):
 
 @plugin.command('tell', 'ask')
 @plugin.nickname_command('tell', 'ask')
-@plugin.example('$nickname, tell dgw he broke something again.')
+@plugin.example('$nickname, tell dgw he broke it again.', user_help=True)
+@plugin.example('.tell ', 'tell whom?')
+@plugin.example('.ask Exirel ', 'ask Exirel what?')
 def f_remind(bot, trigger):
     """Give someone a message the next time they're seen"""
     teller = trigger.nick