Don't fail #getSnapshotDirectories siliently (2/n)

Let's verify the theory in b/170689774#comment4 where the 'after'
list is incorrectly calculated because the 1st #getSnapshotDirectories
fails.

Bug: 170689774
Test: atest StagedRollbackTest
Change-Id: I0570df4642e923d69092bd66125520cb30253863