libsnapshot: handle errors in RemoveAllUpdateState appropriately.

In CheckMergeState / HandleCancelledUpdate, if removing update state
fails, just return the previous state. It used to return Cancelled,
and the error goes unnoticed.

Test: pass
Bug: 147696014
Change-Id: I9cb3d20c5c886afa1913740c903eaad08f0cc041