Exceptions cleanup

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