Add exponential backoff to downloads

[shelvacu]

Sometimes the API thinks you're doing too many requests and so returns an error; continuing to repeat the request many times per second is a great way to make the steam server not like you very much.

This adds a simple exponential backoff when attempting to grab a manifest and when downloading chunks.

It waits 1.3 ^ (attempts so far - 1) seconds up to a maximum of 120 seconds. As a table, the wait times looks like this:

n	time in seconds that it will wait after the nth request
1	1.00
2	1.30
3	1.69
4	2.20
5	2.86
6	3.71
7	4.83
8	6.27
9	8.16
10	10.60
11	13.79
12	17.92
13	23.30
14	30.29
15	39.37
16	51.19
17	66.54
18	86.50
19	112.46
20+	120.00

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Note: The numbers here (min 1s, exponent 1.3, max 120s) were chose somewhat on vibes. They seem roughly right to me, but if you have a better idea I'm happy to change it.