Robustness: Check for broken symlinks to custom actions and complain

[inkarkat]

Before submitting a pull request, please make sure the following is done:

• [X] Fork the repository and create your branch from master.
• [X] If you've added code that should be tested, add tests!
• [X] Ensure the test suite passes.
• [X] Format your code with ShellCheck.
• [X] Include a human-readable description of what the pull request is trying to accomplish.
• [ ] Steps for the reviewer(s) on how they can manually QA the changes.
• [X] Have a fixes #XX reference to the issue that this pull request fixes.