libsnapshot: Add diagnostics for DM_DEV_REMOVE failures.

Example log line:

    update_engine: Block device was lazily unmounted and is still in-use:
        /dev/block/dm-28; possibly open file descriptor or attached loop device.

This will help diagnose bugs such as b/184715543 in the future.

Bug: N/A
Test: manual test
Change-Id: Ia6b17fe9bd1796d59be7fc0b355218509acfd4af