All promise rejections should have the same format

[crantila]

Currently, two types of data may result from a get() or search() Promise that's rejected:

1. If a request is submitted to the Cantus server and the response code is not 200, the Promise is rejected with an object having the following members: code, reason, response.
2. If there is an exception before the request is submitted, the Promise is rejected with a string.

If every Promise rejection used the three-member object described in the first point, error handling in client applications would be easier. We could use zero for the "code" when there's an exception.