don't report webmention failures as task failures

[snarfed]

...in the app engine dashboard. specifically, find an HTTP status code that will make the task queue retry but won't show up as an error request. not sure it exists, but would be nice.

[snarfed]

i'm not sure there's an easy way to do this, at least the way i currently use the task queue. the task queue needs a non-2XX status code to retry, and it looks like those are exactly the codes that are reported as errors on the dashboard.

possible fixes:

• catch all exceptions and enqueue a new task manually instead of returning an error status code
• switch to pull queues

both are nontrivial. :/

[snarfed]

i take it back! looks like the request summary chart only treats 4xx and 5xx as errors. i've switched to 306 in 5f45a5e, which still makes the tasks retry.