Fix missing message id from channel

Sometimes, last_message_id points to a message in a channel that is "not
found". This is OK because we're just figuring out when the last message
was sent, and we use the current time instead.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

discord_markov/client.py

@@ -157,8 +157,13 @@ class Client(discord.Client):
             # NOTE: there is a channel.last_message property, but it's unreliable.
             #       fetch_message(last_message_id) will always yield a
             #       message if there's a valid ID
-            message = await channel.fetch_message(channel.last_message_id)
-            self.set_last_tracked_message(channel.id, message.created_at)
+            try:
+                message = await channel.fetch_message(channel.last_message_id)
+                last_message = message.created_at
+            except discord.errors.NotFound:
+                # sometimes the last message is just not found. What to heck??
+                last_message = util.utcnow()
+            self.set_last_tracked_message(channel.id, last_message)
 
     async def on_message(self, message: discord.Message):
         if message.author.bot: