Issue #s: 963, 640, 946

963:
- Submitting a pull request to make the error message a bit nicer.
- It should also not flood the server with Unhanded exception errors when this triggers.

640:
- Players are not longer able to use negative numbers when using /npc money.
- Instead the absolute value of the amount will be used.

946: Traders inventory can be cleared by everyone.
- Added a configurable 500ms delay to trader right click interaction.
- This prevents the trashing that was occurring where the server thought the player was no longer looking at the trader.