Consider mentioning case-sensitivity in error messages from Discord model converters

### What component of Red (cog, command, API) would you like to see improvements on?

error messages

### Describe the enhancement you're suggesting.

Hi, I've mentioned in #3853 that it's a big UX issue that the role is not case insensitive. This issue also arises in other cogs/any command that uses a role.

I'd like to propose subclassing `RoleNotFound` from discord.py to mention the case-insensitivity, as that's the issue about 90% of the time when users see this error text. It could instead say:

```
Role {role} not found. Please note, roles are case-sensitive.
```

And this would be a much more useful error.

Thanks!

### Anything else?

I'm not sure if this would be an appropriate change to suggest upstream to d.py or not? I haven't opened an issue there, since I *think* this is a Red thing that role inputs are case sensitive?? But I'm really not sure. Any thoughts on this?