Introduce a flow for creating a user. Add error handling.

@bantic for you to review and merge.

This introduces error handling that matches what comes back from the server. Notable around [these lines](https://github.com/mixonic/diesel.aptible.com/compare/aptible:master...mixonic:create-account?expand=1#diff-489df198166b3ae42a99ed05d4a65b32R13).

@fancyremarker the API just seems to return a single error- Ember-Data has support for error messages attached to specific attributes, though I haven't see the API return that yet. Here I'm just creating an omni-present error ghosted where the `message` attribute error would be available.
might be nice to have a `ApiAdapterBase` and `AuthAdapterBase` that we can inherit from, depending on how many models we end up having that reference these endpoints.