Show error message on unsuccessful/invalid command

[ed-george]

Please describe the feature/solution you would like to see
When a user enters an invalid command or a command that returns no results, no acknowledgement of this is shown to the user

Describe how the solution should work
On entering a command that is invalid or returns no results, the bot should reply to the user with a message explaining the error

Describe any alternatives you've considered
Replying the user through a 'reaction' might save on spam

Additional context
N/A