Additional tests from feature/tracks_activation compatible with the older code

These were new tests written of older code, as opposed to the tests written for new code in tracks_activation.

There are a few added constants and exceptions.  Otherwise, this is entirely additional tests.
In the changes to the error handling logic later https://github.com/penzance/ab-testing-tool/pull/20, only errors we specifically mark for display (RenderableErrors) are the ones that a user will see.