Retry network failures when fetching data from FA

[Deer-Spangle]

Retry up to 5 times over up to 3 seconds when trying to fetch a page, if receiving a 502 or 520 error, or a connection reset.

Send the second request right away, but then wait half a second more for each additional request.

Worst case, this will be a total of 3 seconds of sleep. Pseudo timeline:

Attempts = 0
Sleep = 0
--
request
failure
Attempts = 1
A < 5
Sleeping 0
Sleep = 0.5
--
request
failure
Attempts = 2
A < 5
Sleeping 0.5
Sleep = 1
--
request
failure
Attempts = 3
A < 5
Sleeping 1 (total 1.5)
Sleep = 1.5
--
request
failure
Attempts = 4
A < 5
Sleeping 1.5 (total 3)
Sleep = 2
--
request
failure
Attempts = 5
A > 5
raise exception.
--
Total time taken = 3 seconds
Total requests made = 5

So that should be fine in case FA is actually down or unavailable.

In testing for other projects, I've been getting 2-4 of these kind of errors a day, which are simply exposed to users as fa_unknown , not very helpful.
Hopefully this will improve that a bit!