Relic CLI misleading errors

[ModernMAK]

When trying to open a directory as a file permission error is raised instead of a clearer exception.

E.G. relic sga pack v2 "mysga.sga" "mydir" "myconfig.json" will raise PermissionError: [Errno 13] Permission denied: 'mydir' but fails to explain that the issue is because 'mydir' is not a file.

Likewise, 'mysga.sga' should specify that the path is not a directory IF it exists, or that it does not exist.