Mitigate site failures in the event of Gerrit outage

[DanielRyanSmith]

The site was taken down due to failures fetching resources from Chromium Gerrit APIs. Ideally, the site is not taken down entirely when Chromestatus does not have access to Chromium resources.

Steps to mitigate this from happening again:

• Failure to fetch OWNERS file for API Owners does not cause client failures (addressed in When error fetching OWNERS, fall back to stale ndb entity. #4171).
• Origin trial creation form validation does not fail to submit when there is no access to Chromium source file contents.
• Metrics pages do not fail when data is not available (the extent of the cause here might need to be further investigated).