Support multiple errors per test

When using soft assertions (whether ad-hoc or framework-provided), a test may end up with more than one error.

Currently, we have no clean way to handle this. The model and the plugins (both Allure 2 and Awesome) imply one error per test result, which is not ideal because aggregating multiple errors into one won't play well with the future errors view and the error categories feature.

We should:

- Allow the Allure Result JSON to hold multiple errors
- Render all errors of a test result separately

Start by tracing the Allure Result JSON model and the Allure 2 and Awesome plugins mentioned in the issue. Done means the result format can represent multiple errors and the renderer displays every error separately, without aggregating them into one.

Written by the indexing model from the issue text.