Move errors to their own source file.
Move the TokenCreateResult type to its own file.
Screw it exporting.

Otherwise we have no way to *set* those fields.
Authenticate by creating an identity/v3 Client.
Break the import cycle :wrench:

I'll have to bring in an interface if we add functionality to v3.Client for
use in the resource packages.
Make another package for unit testing helpers.
Stub out a provider-level Client for OpenStack.