Capture exception for JSON load in virt.storage_users

If CONF.instances_path/compute_nodes is empty when the storage_users
tries to register the storage use (virt.storage_users.register_storage_use)
the exception is uncaught and raised. As a consequence, the image cache
manager won't run.

Closes-Bug: #1252659

Change-Id: If9ed5e75b0bc0b8f3a7c5fd7ec88f6399e4e59c6
Merge "Capture exception for JSON load in virt.storage_users"