Review error types for `CustomDebugStringConvertible` conformance

Error types that we expect developers to see work better when they conform to `CustomDebugStringConvertible`. The runtime prints a longer error message if the exception scapes `main()`, and the application developer has fewer opaque things to interpret.

Start by locating the repository's error type definitions and checking which developer-facing errors lack CustomDebugStringConvertible. Confirm the runtime output for errors escaping main() and finish when all intended error types provide the clearer debug representation.

Written by the indexing model from the issue text.