Add useful error message in test-mount.py
If /proc/filesystems contains a filesystem that is not listed in MountRule valid_fs, print a useful error message that says what exactly is going on, instead of only saying "False is not True".
If /proc/filesystems contains a filesystem that is not listed in MountRule valid_fs, print a useful error message that says what exactly is going on, instead of only saying "False is not True".