Raise exceptions when encountering errors in files.

The current behaviour of the set_key and get_key functions is to warn for invalid lines, that isn't very helpful when using this library to parse env files

A ignore_errors parameter can be added to these functions

Locate the set_key and get_key functions and inspect how they currently handle invalid lines and emit warnings. Confirm the intended ignore_errors behavior and cover both error-handling paths with tests; done means invalid env-file lines follow the requested exception-versus-warning behavior.