Improve error messages across pipeline stages for better debugging

[JahnaviSingh2005]

Overview

This PR improves error handling and debugging across all major pipeline stages by introducing clear, stage-specific error messages.

Changes

Enhanced error messages in the following modules:

• phase_download.py → Added [Download Phase] context to parquet discovery and checkpoint handling
• phase_extract.py → Added [Extract Phase] context to configuration and initialization errors
• phase_clean.py → Added [Clean Phase] context to Rust extension loading and cleaning failures
• phase_ocr_math.py → Added [OCR Phase] and [Math Phase] context to backend validation and runtime errors
• phase_sections.py → Added [Section Phase] context to metadata updates and input validation

Standardized Error Format

All error messages now follow a consistent format:

[JahnaviSingh2005]

Impact

• Improves debugging clarity
• Provides better context during failures
• Enhances developer and user experience

Scope

• No changes to core logic
• No breaking changes introduced

Additional Notes

Best practices for logging and exception handling are documented in enhanced_error_handling_summary.md.