shake: Produce a result file even if some build failed
GHC HEAD is semi-broken at the moment on the test output of imagine/integrate
due to ghc#19569 (closed). So when I run the benchmark suite, I get a very late report that says as much and then no result file is produced. I'd rather have that the shake build system would produce a measurement file regardless and remember to pass --keep-going
next time I run the benchmark suite.