@ashfurrow => display messages in response to various possible states

I added some error handling for the various steps of registration and bid placement and consolidated those two flows (not in terms of ux, just in how the code is).

* Registration-only, registration and bid, bidding when already registered, bidding + updating a missing credit card -> now all go to the same "loading" view controller which displays some message while requests are made and handles request success/failure before pushing you to the final "bidder details" view.
* Deleted duplicate methods in the "registration" network model (renamed bidder network model, as it fetches your existing bidder without registering you if appropriate) and in the bid placement network model.

 I highly recommend pulling and testing out several different scenarios. I'd also appreciate a second pair of eyes on this -- it probably will need some revision
So there's no `catch` here, so if an error happens, you'll log it, but the error will be propagated up to the subscriber. I'm not sure that's what you want, so you can do this instead with:

```swift
.catch { (error) -> RACSignal in
    log.error("Getting user bidders failed.")
    log.error("Error: \(error.localizedDescription). \n \(error.artsyServerError())")
    return RACSignal.`return`(false)
}
```