allure-behave formatter doesn't generate json files if there is an exception in before_feature() method

When I run behave with allure formatter, and there is an exception in before_feature(), json files doesn't created, which means that when I generate allure report, I get no information about tests in that feature

There is sample project in attachment. Run it with `behave -f allure_behave.formatter:AllureFormatter -o output_folder`
[sample_project.zip](https://github.com/allure-framework/allure-python/files/2729380/sample_project.zip)

- Test framework: behave@1.2.6
- Allure adaptor: allure-behave@2.5.4

Start with the attached sample project and run `behave -f allure_behave.formatter:AllureFormatter -o output_folder` using behave 1.2.6 and allure-behave 2.5.4. Inspect the formatter entry point and existing tests for feature setup failures; done means the failing feature produces JSON files containing information when before_feature() raises an exception.

索引モデルが issue の本文から書いたものです。