fix(nimbus): Update nimbus timeout to 400ms

[vbudhram]

Because

• We were getting timeout errors when calling nimbus experiments

This pull request

• Updates timeout to 400ms (it should take on avg 50ms)
• Only captures sentry event if its an exception
• Emits a statsd metric instead to track timeouts

Issue that this pull request solves

Closes: https://mozilla-hub.atlassian.net/browse/FXA-12711

Checklist

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).

[vpomerleau]

Should we also consider caching the successful responses to reduce the number of fetches?

[vbudhram]

Should we also consider caching the successful responses to reduce the number of fetches?

We could, but I think its ok for now. These failures are very small % of requests.