Add POV on `fatalError()` vs `preconditionFailure()` vs `assertionFailure()`

[tylermilner]

I think our current POV is to limit fatalError() usage to scenarios that are more developer/project configuration related (e.g. it's okay to use when unwrapping baked-in URLs, but not for URLs we get dynamically).

We should also take this article into consideration, which suggests preconditionFailure() would be a better choice over fatalError() due to fatalError()'s potential to leak source file names in release builds.