Less verbose cache miss logging

We try to get cached pages with the 'Cache-Control: only-if-cached' header
set. If pages are not in cache, we'll get a 412 status back.

This patch makes the logging for those cases less verbose.

Change-Id: I844fc43145548c4ffdbc34e9cad921c5c21f2dc6
Merge "Less verbose cache miss logging"