Follow up to #1429. For #695.
This was duplicated in all operations.
`HTTPClient` now only forwards `Result.success` if the request actually succeeded,
leaving each `NetworkOperation` to just parse the body (which will become part of `HTTPClient` too).
Other changes:
- Simplified logic for when to retry requests in `HTTPClient` by using a new `Result.asOptionalResult`
- Removed several response handlers that don't need to do anything anymore
- Removed several tests that were basically duplicated:
- The `finishable` checks are already tested as part of `ErrorResponse`.
- All the checks for the error details are also part of `ErrorResponse`.
- No longer need to have a test for each type of failed response since that's part of `HTTPClient` tests now.
NachoSoto