Improve error messages in handlers

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

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

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

Start by locating the handler implementations under lib and reviewing their existing error paths and messages. Determine which errors need more context, then verify that the resulting messages improve debugging and maintenance without changing the underlying behavior.

Written by the indexing model from the issue text.