Avoid loading an empty token when decrypt failed

This CL avoids loading an empty token when the decrypt fails. It also
add logs and user metrics for errors during the load of the tokens
from the DB in order to measure if there are platforms on which this
occurs repeatedly.

BUG=686485

Review-Url: https://codereview.chromium.org/2672603003
Cr-Commit-Position: refs/heads/master@{#448609}
3 files changed