Add retries for Sentry monitor API calls

[bchopson]

Refs GH-192

[bchopson]

I considered addressing some of the concerns regarding the ergonomics of config variables and such, but couldn't come up with a better approach that seemed worth breaking existing usage.

[rsyring]

@calebsyring you opened, and commented repeatedly on :), #192. Any thoughts?

[calebsyring]

@calebsyring you opened, and commented repeatedly on :), #192. Any thoughts?

"Refs" in the original comment is correct. There are concerns on that issue that this pr doesn't address, but what it does will certainly be helpful.

FWIW I have noticed that Sentry seems to go down in blocks of time (which is when it spits out these various error codes). This PR will help reduce spam from failed pings, but more sensitive monitors may still fail if their pings don't go through. This doesn't have any bearing on the pr, but I wanted to put out there that it may not be feasible to use Sentry for more precise monitors due to this unreliability (or we may have to build some more margin into our configurations).