First specs for error reports working...

Modified SudokuValidator methods to return error values and convert to strings for reporting
CLI program not printing those results yet
have some duplication to be taken care of before that step.
Refactor: rename report_errors to identify_errors
Refactor validity-checking to remove duplication...

report_errors now a separate method to generate error strings.
Refactor: clean up report_errors usage
Complete refactoring...

Duplication removed from check_<element>_validity methods.