Add RSpec matchers for testing json_api error responses

[swilgosz]

There is a lot of tedious work involved when there is a need to test JSON:API endpoints. All endpoints can send successful and unsuccessful responses and in terms of possible errors a lot of repetition is involved.

Suggestion:

Add an RSpec matcher to easily check the error responses, i. e:

expect(json).to be_json_api_error(:not_found)
expect(json).to be_json_api_validation_error(validation_errors)

We could add some arguments to it to allow for customization of the expected messages.