libext2fs: don't ignore fsync errors

libext2fs: don't ignore fsync errors

Today, if mke2fs experiences IO errors (say, on a thin device
which filled up during mkfs), mke2fs is silent and returns
success even though the filesystem was not properly created.

Catch errors from the io_channel_flush() callchain to
fix this up.  Fix formatting of the printed error as
well:

...
Creating journal (262144 blocks): done
Writing superblocks and filesystem accounting information:
Warning, had trouble writing out superblocks.
# echo $?
5

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>