Fix LOG string formatting in image_utils

My recent change to report stats on image fetch and
conversion used an incorrectly formatted log message.
The result is Trace messages with Key errors in the c-vol
log files.

This change fixes those messages and gets rid of the key error
exception messages in the Logs.

Change-Id: Id8377dd7761d93fd854f5f7871c997418c6e4554
Closes-Bug: 1357726
Merge "Fix LOG string formatting in image_utils"