diff should not ignore errors

[aptituz]

If diff is failing for any reason (e.g. because of wrong arguments when providing them via diff_options) the diff method is silently failing, returning no diff but no error message either. That's probably because it's not evaluating stderr nor return code in the block where diff output is being processed.

[mainameiz]

Fixed in https://github.com/samg/diffy/pull/98