Create structured exception hierarchy with exit codes

Implements comprehensive exception hierarchy for better error handling,
structured exit codes, and retry support for transient failures.

New file: src/vlm/exceptions.py

Exception Classes:
- VLMError (base, exit code 1)
  - VLMConfigError (exit code 2) - configuration errors
  - VLMFileSystemError (exit code 3) - file operations
  - VLMIOError (exit code 4) - data file I/O
  - VLMTransientError (exit code 1, retryable) - temporary failures
  - VLMValidationError (exit code 5) - data validation
  - VLMQuarantineError (exit code 6) - quarantine operations

Features:
- Structured exit codes for different error types
- is_retryable flag for transient errors
- Clear error messages with context
- Ready for future retry logic implementation

Usage Example:
```python
try:
    validate_config(config)
except VLMConfigError as e:
    sys.exit(e.exit_code)  # Exit with code 2
```

Future Work:
- Migrate existing ValueError/Exception usage to new hierarchy
- Implement retry logic for VLMTransientError
- Add exit code handling in CLI main()
- Add error context tracking

Testing:
- Module loads successfully
- Exit codes and flags verified
- All 449 tests still pass

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/vlm/exceptions.py

@@ -0,0 +1,120 @@
+"""Exception hierarchy for Video Library Manager.
+
+Provides structured exceptions with exit codes and retry support for better
+error handling and user experience.
+"""
+
+
+class VLMError(Exception):
+    """Base exception for all VLM errors.
+
+    All VLM-specific exceptions inherit from this class.
+
+    Attributes:
+        exit_code: Suggested exit code for CLI applications (1 by default)
+        is_retryable: Whether the error might succeed if retried
+    """
+
+    exit_code: int = 1
+    is_retryable: bool = False
+
+    def __init__(self, message: str, *args, **kwargs):
+        """Initialize VLM error.
+
+        Args:
+            message: Error message
+            *args: Additional positional arguments for Exception
+            **kwargs: Additional keyword arguments
+        """
+        super().__init__(message, *args)
+        self.message = message
+
+
+class VLMConfigError(VLMError):
+    """Configuration-related errors.
+
+    Raised when configuration is invalid, missing required fields, or cannot be loaded.
+
+    Examples:
+        - Missing library_root
+        - Invalid YAML syntax
+        - Invalid category mappings
+    """
+
+    exit_code = 2
+    is_retryable = False
+
+
+class VLMFileSystemError(VLMError):
+    """File system operation errors.
+
+    Raised when file operations fail (read, write, move, delete).
+
+    Examples:
+        - Permission denied
+        - File not found
+        - Disk full
+    """
+
+    exit_code = 3
+    is_retryable = False
+
+
+class VLMIOError(VLMError):
+    """I/O errors for reading/writing data files.
+
+    Raised when loading or saving inventory, identities, analysis, plans, etc.
+
+    Examples:
+        - Invalid JSON format
+        - Schema version mismatch
+        - Missing required fields
+    """
+
+    exit_code = 4
+    is_retryable = False
+
+
+class VLMTransientError(VLMError):
+    """Transient errors that may succeed if retried.
+
+    Raised for temporary failures that might resolve on retry.
+
+    Examples:
+        - Network timeouts (TMDB API)
+        - Temporary file locks
+        - Rate limiting
+    """
+
+    exit_code = 1  # No special exit code (will retry)
+    is_retryable = True
+
+
+class VLMValidationError(VLMError):
+    """Data validation errors.
+
+    Raised when data fails validation checks.
+
+    Examples:
+        - Invalid filename patterns
+        - Invalid season/episode numbers
+        - Invalid quality metrics
+    """
+
+    exit_code = 5
+    is_retryable = False
+
+
+class VLMQuarantineError(VLMError):
+    """Quarantine operation errors.
+
+    Raised when quarantine or restore operations fail.
+
+    Examples:
+        - Cannot quarantine file from unsupported category
+        - Quarantine manifest corruption
+        - File already in quarantine
+    """
+
+    exit_code = 6
+    is_retryable = False