Reorganise the ClientError enum.

Reorganise the ClientError enum.

Now it has lots more cases, which should make it easier to format each
one nicely into an entry for a TUI-displayed error log.

Other changes:

 - when we keep the error message from the JSON error document, we
   _also_ keep the HTTP status code

 - lots of from_request calls were missing (possibly in the stuff I
   moved in here from the old separate login.rs). Reinstated.