Fix error behaviour in mini-benchmark

In commit f5ca4077, an the "--log" option was added but it was implemented by
invoking a sub-shell. Any errors in the sub-shell would not be propagated and
thus cause mini-benchmark to suppress errors.

The fix here is to propagate the exit status of the sub-shell.

All new code of the whole pull request, including one or several files that are
either new files or modified ones, are contributed under the BSD-new license. I
am contributing on behalf of my employer Amazon Web Services, Inc.

support-files/mini-benchmark.sh

@@ -310,4 +310,4 @@ case $RESULT in
 esac
 # Record the output into the log file, if requested
 ) 2>&1 | ($LOG && tee "$BENCHMARK_NAME"-"$TIMESTAMP".log)
-
+exit ${PIPESTATUS[0]} # Propagate errors in the sub-shell