Avoid deleting reminders silently

[wookie184]

Currently a reminder will be ignored+deleted if the bot cannot get the channel it was meant to reply in: https://github.com/python-discord/bot/blob/324cb65b5e2fbc52cb494f02a11dc40e56f0a5cb/bot/exts/utils/reminders.py#L112-L124

This could happen if the remind command was run in a thread, or a channel that was deleted.

There are a couple of options here:

• Don't allow the command in threads, or always send the reminder to e.g. #bot-commands
• Send the reminder to e.g. #bot-commands if it failed to send. Is it ok to send reminders that may have been run in a private channel to a public channel though?
• Try to DM the reminder to the user(s) - DMs may not always be open though
• Just tell them the reminder failed to send in #bot-commands - avoids ever leaking anything and is simple, but they don't get the contents.

This is an edge case so I think whatever the solution it should be as simple as possible, just something better than not reminding the user(s) at all.