Handle BlobNotFoundError in /restricted/taskqueue/verify/.
Handle BlobNotFoundError in /restricted/taskqueue/verify/.

Some funny guys could have deleted the blob before it could even be verified.
Make sure the server doesn't crash and property delete the corresponding
ContentEntry entry.

R=csharp@chromuim.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/15889010

git-svn-id: 8fa0a038c61565a87d0885d584517a2e12819dac@202556 0039d316-1c4b-4281-b951-d872f2087c98

madecoste/swarming,madecoste/swarming,luci/luci-py,madecoste/swarming,pombreda/swarming,pombreda/swarming,pombreda/swarming,luci/luci-py,luci/luci-py,madecoste/swarming,pombreda/swarming,luci/luci-py