tools: preserve make errors, ignore empty output

tools: preserve make errors, ignore empty output

Exit codes lost since commit v1.9-305-ga7e3d17ab8 (2017-05-29)
[tools: atomic creation of include files].  Fixed using `mispipe`
also provided by moreutils.  As additional fail-safe, also catch
empty results with `ifne` to keep existing data on failure.