Fix incorrect None check in react_roles - use proper exception handling

[Claude]

Discord.py's fetch_message() never returns None - it either returns a Message object or raises exceptions (discord.NotFound, discord.Forbidden, discord.HTTPException). The code incorrectly checked if not message: after fetch calls, preventing proper error handling.

Changes

• add_react command (lines 55-65): Replace None check with try-except blocks
• remove_react command (lines 97-107): Apply same exception handling pattern

Before

message = await channel.fetch_message(message_id)
if not message:  # Never triggers - fetch_message raises exceptions
    await ctx.send("That message doesn't exist anymore.", ephemeral=True)
    return

After

try:
    message = await channel.fetch_message(message_id)
except discord.NotFound:
    await ctx.send("That message doesn't exist anymore.", ephemeral=True)
    return
except discord.Forbidden:
    await ctx.send("No permission to access that message.", ephemeral=True)
    return
except discord.HTTPException as e:
    await ctx.send(f"Failed to fetch message: {e}", ephemeral=True)
    return

This prevents unhandled exceptions when messages are deleted and matches the pattern used in party/party.py and game_embed/game_embed.py.