Catch exception when image cache pruning

If a cached image file does not exist, os.stat will throw an exception
and the glance-cache-pruner can fail to run when it encounters
the non-existent file.

Adds a catch for the possible exception and sets file size to 0 so
the glance-cache-pruner can successfully run and prune the cache.

Change-Id: I55bee82c4a7aa0393d36c603c91adb8b44a7c7a3
Fixes: bug #1286240
Merge "Catch exception when image cache pruning"