ProcessorCount test: require SystemInformation process to work

Currently this silently fails on some systems. Make sure those things get
noticed so we can fix that.

Tests/CMakeTests/ProcessorCountTest.cmake.in

@@ -12,7 +12,14 @@ execute_process(
   COMMAND "${KWSYS_TEST_EXE}"
   testSystemInformation
   OUTPUT_VARIABLE tsi_out
-  ERROR_VARIABLE tsi_err)
+  ERROR_VARIABLE tsi_err
+  RESULT_VARIABLE tsi_res
+)
+if (tsi_res)
+  message("executing \"${KWSYS_TEST_EXE}\" failed")
+  message(FATAL_ERROR "output: ${tsi_res}")
+endif ()
+
 string(REGEX REPLACE "(.*)GetNumberOfPhysicalCPU:.([0-9]*)(.*)" "\\2"
   system_info_processor_count "${tsi_out}")