BE - nested_session hides exceptions for unit tests

[`nested_session()`](https://github.com/bcgov/lear/blob/feature-legal-name/legal-api/tests/unit/__init__.py#L41) is currently used in unit test at BE (legal-api, filer, emailer etc.). It ignores exception as a result real test logic can be skipped and problems can be hidden.

**Example (Legal API)**
- wrong version
mandatory field `start_date` is missing and it returns as success before the following real test logic is executed

- correct version