helpers.py: check subprocess exit status in readpipe().
helpers.py: check subprocess exit status in readpipe().

Checking the exit status seems like a good idea.  Note that we can't
just use check_output() because it wasn't added until Python 2.7.

Thanks to Alexander Barton <alex@barton.de> for reporting the version
incompatibility with respect to an earlier version of this patch.

Signed-off-by: Rob Browning <1dc5917c2adeeba13d1ef33dbb40691a1b67cc65@defaultvalue.org>

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