Fix error raised by bad z!duel argument

and also broaden the scope of exception types that display the "Format: x" message, rather than being thrown

bot/game.py

@@ -304,6 +304,8 @@ async def duel(ctx: commands.Context, opponent: User):
         raise commands.CommandError("You can't challenge yourself.")
     if opponent.bot:
         raise commands.CommandError("You can't duel a bot.")
+    if opponent not in ctx.guild.members:
+        raise commands.CommandError("User is not in this server.")
 
     if not await confirm(f"{opponent.display_name}, do you accept the challenge?", ctx, opponent,
                          emol=du, yes="accept", no="chicken out"):

bot/main.py

@@ -361,7 +361,7 @@ async def on_command_error(ctx: commands.Context, exception):
     if ctx.command == epitaph and ctx.channel in zeph.epitaphChannels:
         zeph.epitaphChannels.remove(ctx.channel)
 
-    if type(exception) in [commands.MissingRequiredArgument, commands.BadArgument, commands.TooManyArguments]:
+    if isinstance(exception, commands.UserInputError):
         await err.send(
             ctx, f"Format: `{'` or `'.join(ctx.command.usage.splitlines())}`"
         )