Separate unsafe flow exceptions into user/server errors

### Context

Currently, all exceptions thrown as part of the unsafe flow process are categorized as "server errors" (or just "Failure") in the subscription trigger history. Some cases of unsafe flow exception are clearly a repo / codeflow configuration issue, and not an error in the processing itself.

### Goal

Turn all the unsafe-flow related configuration exceptions into "User errors", and make sure they have clear and helpful exception messages attached.

Start by tracing unsafe-flow exception handling into subscription trigger history and identify which configuration exceptions are currently categorized as server errors. Review the existing exception messages and classification paths; done means configuration-related unsafe-flow failures appear as user errors with clear messages, while processing failures remain server errors.

Written by the indexing model from the issue text.