retain existing exception when raising

[leohemsted]

this'll enable us/users to interrogate the original raw request exception if they need more context

What problem does the pull request solve?

Checklist

• [x] I’ve used the pull request template
• [ ] I’ve written unit tests for these changes
• [ ] I’ve updated the documentation in
  • [ ] notifications-tech-docs repository
  • [x] CHANGELOG.md
• [x] I’ve bumped the version number in
  • [x] notifications_python_client/__init__.py
• [ ] I've added new environment variables to
  • [ ] notifications-python-client/scripts/generate_docker_env.sh
  • [ ] notifications-python-client/tox.ini
  • [ ] CONTRIBUTING.md