Put back the error checking in wtfs() that was lost when newfs was
changed to use libufs in revision 1.71. Without this, any write
failures in newfs were silently ignored.

Note that this will display a meaningless errno string in the case
of a short write as opposed to a write error, since bwrite()'s
return value does not allow the caller to determine if errno is
valid.

Reported by: Lukas Ertl <l.ertl@univie.ac.at>
Reviewed by: jmallett
Approved by: re (bmah)