Call perror on popen failure

If popen fails in pipe_read_line we invoke perror for the error message, and
pipe_read_line is in turn called by find_other_exec which is used in both
frontend and backend code.  This is an old codepath, and it seems like it ought
to be replaced with the more common logging tools we now have like in the
attached diff (which also makes the error translated as opposed to now).  Any
objections to removing this last perror() call?

--
Daniel Gustafsson