ctest_memcheck: Change failure to find log file from error to warning
Sanitizers do not create a log file when no defects are found. Therefore, it is currently impossible for ctest_memcheck to set both `CAPTURE_CMAKE_ERROR` and `RETURN_VALUE` to zero. With defects, `CAPTURE_CMAKE_ERROR`=0 and `RETURN_VALUE`=-1, as expected. With no defects, `CAPTURE_CMAKE_ERROR`=-1 and `RETURN_VALUE`=0.