U571 - access denied message

U571 - access denied message
Since only developers know that "13" is EACCES, try to include the text
message.  I'm not 100% sure the ifdefs are complete, but it can be tweaked
as needed.  This was the only common message in files.c that included errno
so it's the only one I changed.  Of course, "13" is only one of several
possible errno values that might reasonably show up here.