[fertiscan] As a dev, I want to receive a proper error message when deleting a non-existent inspection

## Description

**Context**
When attempting to delete an inspection that was already deleted, the system returns a database error:

```
Inspector is not the creator of inspection
CONTEXT: PL/pgSQL function delete_inspection(uuid,uuid) line 11 at RAISE
```

**Problem Statement**
Instead of this database error, the system should return such error message as "Inspection not found."

## Acceptance Criteria

- Attempting to delete a non-existent inspection should return an "Inspection not found" error.