Revamp login so it can be used to probe prospective hosts for legitimate
SpiderOak service, ensuring that settings are not changed without success.

Make both the success and error callbacks include the xhr data, so
that the error response can be examined for conformance to behavior expected
of a SpiderOak server.

Move the change of the server-host specific login location settings to the
success, not before!
Revamp the handling of the server host change, in order to validate.

We dance around a bit to do that and also logout, if necessary:

- We have to go through the error (and possibly success) callback on
  a login test to verify that the new designated address host a valid
  SpiderOak service. (We use deliberate deliberately invald login credentials.)
- We use a further callback, for employment via logout in the case
  that server change is valid and the current session is authenticated.