Autocomplete custom no results

[muddyfish]

For certain slash commands, I've got autocompletes which have no useful options for certain users, even with no input. It would be nice to tell the user why there's no results.

Example commands:

• A sell item command when you have no items
• Delete emoji command in a server with no emojis

[Mehgugs]

I think in these cases, a proposal like #4615 would solve this problem more generally; for example signalling an error from auto-complete if there's an issue – like there being no emojis to pick from etc.