Improve error messages in controllers

**Problem**
When working with handlers, 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 controller and handler error paths in the Shell-based terminal agent and reviewing how current errors are produced. Identify the cases that lack useful context, then define descriptive messages for those paths and verify that debugging information is improved without changing the underlying behavior.

Written by the indexing model from the issue text.