Error handling

[kutsurak]

If the data generation failed for any reason, (for instance make is not installed) subsequent invocations of tpch_build.sh will fail as well.

In this specific case the problem is that the build script checks if the directory 02_load/SF-xxx exists, not that it actually contains data.

The solution to this would probably be to fail earlier, and thus avoid creating that directory in the first place, but a more holistic approach to error handling might be desirable.