Avoid dangling partial image on size/checksum mismatch

Fixes bug 1122299

Previously, when the supplied image size or checksum didn't match
the calculated values, partial image image data was left dangling
in the backend store.

Now, we clean up when the mismatch is detected.

Change-Id: I916d78ef3b2065e87df31ec4fb405915417d68aa
Merge "Avoid dangling partial image on size/checksum mismatch" into stable/folsom