Most TaskFlow exception don't have 'Exception' suffix in their names, so
we drop it from the few that have it, for sake of consistency and more
concise and expressive code.
MissingDependencies parent object is changed from InvalidState to
InvariantViolation, which is more appropriate.
Unused ClosedException is dropped.
Breaking change: any client code that used renamed exceptions,
ClosedException or relied on catching MissingDependencies as
InvariantViolationException is affected.
Change-Id: I649b7760d382ccb6df333bdffd667ba6b67a431e
9d0b16e7fb