Previously failed API calls from the `store` module were rejected with a
{ status, statusText, data, headers } response object.

This had several problems:

 * This violated the convention that promises should be rejected with
   Error or Error-like objects, so callers could not rely on the result
   having a `message` property.

 * It was up to each of the callers to translate the result into
   a message suitable for display, which is likely to lead to
   inconsistency.

 * Testing the translation of different kinds of failures into
   messages was mixed up in the `<annotation>` component tests.

This commit moves the translation of $http responses to `Error` objects
into the API client (`store`) module and adds tests.