Add HTML export and cell execution checking to nbexec

[janpfeifer]

This pull request adds two new features to the nbexec tool:

1. -export_html flag: Exports the notebook as HTML to a specified destination path after execution. Headless Chrome is configured to download the HTML to a temporary directory first, avoiding path pollution, and then moves the file to its final target path (with fallback copy-handling across different filesystems).
2. -check_cells flag: Parses the executed .ipynb file after completion and checks for any compilation errors (via output_type: "error") or runtime errors/panics (via checking stderr for exit status or panic:). Prints a detailed error report of failed cells and exits the nbexec process with exit code 1 if any failures occurred.
3. Added hover context for execution errors (similar to what was already there for compilation errors); Hover context also activated over the cell number.
4. Fixed layout issues on error HTML output.

Additionally, this PR adds a new test notebook at examples/tests/errors.ipynb and implements the corresponding integration test TestErrors in internal/nbtests/nbtests_test.go to verify these error-matching behaviors and line numbers.