Add custom exception hierarchy for better error handling

[NP-compete]

Problem

The codebase uses generic Exception for all error cases:

raise Exception("Processing failed")  # processor.py:291
raise Exception(f"finish reason: {finish_reason}")  # processor.py:440

This makes it difficult to:

• Catch specific error types
• Provide meaningful error messages to users
• Debug issues in production

Proposed Solution

Create a custom exception hierarchy in src/pageindex/exceptions.py:

class PageIndexError(Exception):
    """Base exception for all PageIndex errors."""
    pass

class DocumentParseError(PageIndexError):
    """Raised when document parsing fails."""
    pass

class TOCExtractionError(PageIndexError):
    """Raised when TOC extraction fails."""
    pass

class LLMResponseError(PageIndexError):
    """Raised when LLM returns invalid or unexpected response."""
    pass

class ConfigurationError(PageIndexError):
    """Raised when configuration is invalid."""
    pass

class ConversionError(PageIndexError):
    """Raised when document conversion fails."""
    pass

Acceptance Criteria

• Create exceptions.py with exception hierarchy
• Replace all raise Exception(...) with specific exception types
• Replace bare except Exception with specific catches where appropriate
• Add exception documentation to README
• Add tests for exception handling

Files to Modify

• Create src/pageindex/exceptions.py
• src/pageindex/pdf/processor.py
• src/pageindex/pdf/toc.py
• src/pageindex/markdown/processor.py
• src/pageindex/batch.py
• src/pageindex/llm.py