Typing /help + unavailable command returns "unexpected error" message

The bug

Typing /help and a command that's not available (either because it doesn't exist, or because its usage is not allowed, e.g. when cheats are disabled) returns An unexpected error occurred trying to execute that command, which generates an exception on the game's log. This should be a less severe "Unknown command" error message.

Note

Prior to 18w02a this would return java.util.NoSuchElementException. It now displays a nicer message as a result of fixing MC-121662.