Improve authentication and http/json failure result messages

This is an attempt to centralize message reporting. I think it makes things a lot better. It should go in after https://github.com/warmshowers/wsandroid/pull/146 gets pulled.

Questionable things:

* Putting the (static) message creator into RestClient.reportError() is questionable. I didn't see a better home for it. It's much better than having completely different handling everywhere.
* This makes all of the auth/http/network handling use Toast messages. In one place there was a dialog instead (and there's still a dialog on success). 

Of course with code that's getting older (and now that has 2 devs) there is lots of inconsistency in HTTP handling. I almost bit it off but instead spun it off into https://github.com/warmshowers/wsandroid/issues/147