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).
Current behaviour when we issue waring but still proceeed and
return success is 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. See
linked DR for details.

MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D18584