Add more test coverage for handlers

Rework test interface implementations to be configured using variadic
functional configuration. Simpler client code.

Use Contexts more, and extract out common DataSetStorage set up.
Correct pass-through of variadic argument
Change error response format

- add status and message field
- allow multiple errors to be rendered
- add Gomega Matcher for checking HTTP API responses
- DRY tests
Merge pull request #17 from alphagov/more-handler-tests

Add more test coverage for handlers