Fix KeyNotFoundException for nonexistent vents (#413)

Fix KeyNotFoundException for nonexistent vents (#413)

Some mods add extra vents that don't yet exist, this commit checks if a
given vent exists and if it doesn't, throw an AC violation instead of
disconnecting the player

One downside is that the Enter/ExitVent events are no longer called, but
this would require the vent in that event to become nullable