Review how we handle error codes

[mrcnski]

We should know the StatusCodes we should receive and handle them accordingly. Something like a switch statement that handles different codes explicitly and gives a developer error if we have missed one?

Otherwise if that is over the top I think we should explicitly check for success and return an error if not so we aren't hiding any weird status codes that might create UX bugs or inconsistencies for users.

See https://github.com/NebulousLabs/go-skynet/pull/10#pullrequestreview-446519862

I'm thinking we treat >= 200 and < 300 as success and anything else as unexpected and therefore an error.

[ro-tex]

We need to treat 3xx according to their specification - following redirects, etc. We can choose to return them to the caller, in case the API supports that or we decide that it should support it. But we can't treat a redirect as an error, especially now that siad is using redirects in order to ensure skylinks have a trailing /.

[mrcnski]

Seems like Go's http client will follow redirects automatically, so there's nothing we need to do there.

[ro-tex]

In that case I'll close the issue.

[ro-tex]

I misread. We don't need to do anything special for the 3xx case but we still need to do that rest.