Show error when trying to delete a list that doesn't exist

When trying to delete a list that doesn't exist, boom currently still says it was deleted successfully:

```bash
$ boom delete my_cool_list
You sure you want to delete everything in my_cool_list? (y/n): y
Boom! Deleted all your my_cool_list.

$ boom delete my_cool_list
You sure you want to delete everything in my_cool_list? (y/n): y
Boom! Deleted all your my_cool_list.
```

This means I'm not sure whether boom actually just did anything or not.

I've added a check to make sure the list actually exists, and to print an error message if it doesn't:

```bash
$ boom delete my_cool_list
You sure you want to delete everything in my_cool_list? (y/n): y
Boom! Deleted all your my_cool_list.

$ boom delete my_cool_list
We couldn't find that list.
```