Added solution to Project Euler 69 (#2934)

* Added solution to Project Euler 69

* Accept edits from code review

Co-authored-by: Dhruv <dhruvmanila@gmail.com>

* Added doctests

* Renaming and exception handling

* Apply suggestions from code review

Co-authored-by: Dhruv <dhruvmanila@gmail.com>

* Edited mistake.

Co-authored-by: formal-acc <sarthak.agrawal@research.iiit.ac.in>
Co-authored-by: Dhruv <dhruvmanila@gmail.com>