safe_fork: Improve the error message

[sjackman]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew tests with your changes locally?

[apjanke]

👍

[MikeMcQuaid]

Thanks again @sjackman!