libsnapshot: Allow inspect_cow to show failed blocks

Adds the -b option to show the bad data block that failed to decompress.
If the block is large enough, display the front as though it were a
CowOperation, as this is the most likely culprit.

Change-Id: I287f13e0794a1ca9d647d4b1099ab238a6202b23
Bug: 183985866
Test: inspect_cow -db <COW_FILE>