CM11 Themes: Reduce "unable to load asset" log spam

With the introduction of a "common" resource folder for themes,
we always try to attach the common resource table and if it does
not exist it won't be attached but we would see this log spam.

Change-Id: Ida4e815523fd71b34fe8f3b41cef3432e6e07b64
CM11 Themes: Allow defining a custom default theme [1/3]