Abort analyse step when parse errors occur

[jordansilly77-stack]

Summary

• stop the analyse step when parser errors are returned
• reuse the existing check_for_errors helper for consistent step behaviour
• update the unit test to assert parse errors halt the build

Closes #577.

Tests

• .venv/bin/python -m pytest tests/unit_tests/steps/test_analyse.py tests/unit_tests/steps/test_steps.py
• .venv/bin/python -m flake8 source/fab/steps/analyse.py tests/unit_tests/steps/test_analyse.py

I also ran .venv/bin/python -m pytest tests/unit_tests/steps; unrelated test_find_source_files cases fail locally because no Fortran compiler is available.

[Matthew Hambley (MatthewHambley)]

This looks like a useful change but your raising it pointed out that we didn't have a contributors agreement in place. We now do so you'll need to deal with that before this change can progress.