Dog generator: Make responses ephemeral

[NMertsch]

Currently, all users see the dog generator's error responses. Let's use ephemeral messages here, like we do in the registration setup. This would also make it tolerable to provide feedback when the user is rate-limited or posting in the wrong channel.

A not-so-good alternative is using delete_after=5 to auto-delete the messages after 5 seconds, or to not send error messages at all.

@nekeal do you want to look into this?

[nekeal]

I tried to use ephemeral messages for informing about the rate limit but even with ephemeral=True, the message was visible to anyone. I found the following info in the discord.js docs.

Regarding making the error messages ephemeral, my intention was actually the opposite. I wanted them to be funny so that, in case of an API error or misconfiguration, everyone can immediately see that the feature is unavailable instead of having to check it themselves.