Sentry: Creation of this event was denied due to rate limiting

[rfay]

Describe the bug

We were rate-limited today on Sentry and a user got
"raven: got http status 429 - x-sentry-error: Creation of this event was denied due to rate limiting"

This kind of note should not be shown to the user, and certainly should not cause operational error.

I think the user was actually dealing with a different problem and this was just noise, but still, it should not be shown.

[rfay]

I haven't actually been able to figure out any place in our code that would have emitted this, so assume that raven-go emitted it. #1724 will prevent some of this kind of thing, but like I say, I haven't found how this could have come from our code.

Note that we authorized Sentry to bill us and prevent this kind of thing.