Require exception classes

We were referencing the GdsApi::BaseError class in our code, without explicitly
requirin 'gds_api/exceptions'. In development mode this wasn't a problem, because
an adapter class, which does require it, was always loaded first. When we deployed
to preview however, the application couldn't boot because the constant was
undefined.
Merge pull request #189 from alphagov/require_exceptions_classes

Require exception classes