MFC r344555 (sobomax): makefs: Improve error handling

Bail out if one of the files scheduled to go to the FS image we are
making cannot be read (e.g. EPERM). Previous behaviour of issuing a
warning but still proceeeding and returning success was definitely not
correct: masking out error condition as well as making a slighly
inconsistent FS where attempt to access the file in question ends up in
EBADF. (More details in review D18584.)