Add a message for invalid sublevel parameters - Fixes BUKKIT-4860

The Issue:
A message is not sent for a invalid /scoreboard parametre, example
          /scoreboard players stuntguy3000

Justification for this PR:
To alert players that they used invalid syntax

PR Breakdown:
Adds an else statement to the checks for valid syntax

https://bukkit.atlassian.net/browse/BUKKIT-4860