Add emoji indicators to the bot's responses

[TimJentzsch]

This PR adds emojis to the bot's responses to make it more clear when something has gone wrong:

• A checkmark if the command has been executed successfully.
• A warning sign if something looks off.
• A no entry sign if the command failed.
• Some misc emojis for other things.

I also rephrased some responses and added a note about the help command in the error cases.