Improve Job Expansion & Better Error Handling

Provides a handful of changes to improve the job_expand() method in variables.rs, and some improvements to the shell_expand module in regards to error handling. Errors are now captured in the words.rs module instead of in the braces.rs module, which was causing a bug with improperly detecting some unmatched braces. Error messages will now also fully print the original command and correctly point out the location of the unmatched brace. An additional unit test was also added for the shell expand module.