Update middleware documentation

**Problem**
When working with parsers, the error messages could be more descriptive.

**Suggestion**
Improve error handling and add detailed error messages.

**Context**
This would help with debugging and maintenance.

No files, entry points, or tests are named. Start by locating the middleware and parser error-handling paths, then determine which failures currently produce vague messages; done means the relevant parser errors are more descriptive and the behavior is covered by appropriate tests.

Written by the indexing model from the issue text.