Fix the Finder's ability to ignore permission errors
It turns out the os was returning a different permissions error than
os.ErrPermission. Fortunately we can just use os.IsPermission().
Bug: 62455338
Test: mkdir not-permitted && chmod 000 not-permitted && m nothing
Change-Id: Ieb1653b9687ffdaae83184dcdcd4166ddb6a4613