cmd/fsck: correctly catch nonzero return codes of 'par2 create'.
cmd/fsck: correctly catch nonzero return codes of 'par2 create'.

Oops; we weren't checking the return value like we should.  Reported by
Sitaram Chamarty.

jbaber/bup,ToxicFrog/bup,tjanez/bup,mhoeher/bup,apenwarr/bup,tjanez/bup,tjanez/bup,ToxicFrog/bup,jbaber/bup,pombredanne/bup,tjanez/bup,apenwarr/bup,jbaber/bup,pombredanne/bup,mhoeher/bup,ToxicFrog/bup,ToxicFrog/bup,apenwarr/bup,jbaber/bup,mhoeher/bup,tjanez/bup,jbaber/bup,mhoeher/bup,pombredanne/bup,gevaerts/bup,pombredanne/bup,gevaerts/bup,pombredanne/bup,apenwarr/bup,mhoeher/bup,gevaerts/bup,gevaerts/bup,ToxicFrog/bup,pombredanne/bup,gevaerts/bup,jbaber/bup,tjanez/bup,mhoeher/bup