bug#9574: data-loss with --batch: ignored write failure

[Jim Meyering]

This should diagnose the write failure and exit nonzero:

    emacs --version > /dev/full

Currently (latest from bzr trunk as of yesterday),
emacs exits 0 (successfully) and gives no diagnostic.
Obviously that doesn't matter too much, but this next one
can easily cause real trouble:

    $ emacs --batch --eval '(print "important")' > /dev/full && echo FAIL
    FAIL

I.e., the write fails and emacs fails to report that failure.

That means anything written via --batch to standard output may
be corrupted or incomplete, and emacs will still exit successfully.