Don't 500 if we can't store the builders json in memcache.

The builders json is over the 1MB memcache limit and the flakiness
dashboard is now failing to serve at all. With this "fix" it loads
after the ~45 seconds it takes to generate the builders json.
Hopefully this doesn't kill buildbot.

BUG=261027
NOTRY=true
R=mmontgomery@chromium.org

Review URL: https://codereview.chromium.org/19526002

git-svn-id: svn://svn.chromium.org/blink/trunk@154546 bbb929c8-8fbe-4397-9dbb-9b2b20218538