[RFC] build: print the error result when the tests fail

Any diagnostic information is useful when things fail.  In my case, it
printed out the fact that the tests were segfaulting.
> RESULT_VARIABLE
    The variable will be set to contain the result of running the processes. This will be an integer return code from the last child or a string describing an error condition.

ret is not only the return code? Never would have guessed.. We should try that for GenerateHelptags as well, when that fails there is usually no error message at all.