Check for process errors in make_qbk.py

[Daniel James]

The script doesn't check for errors when building the documentation in make_qbk.py, so my documentation build script doesn't notice that anything is wrong. As you might recall this resulted in some of the documentation failing to build in the last release.

I've attached a patch for a possible solution.

[Barend Gehrels]

Thanks! Will apply it.

[Barend Gehrels]

Patch is applied.