preprocessing, extraction and projection will not stop if files are missing

When there is an issue preprocessing the file or extracting/projecting features and the tool returns None, a message is written, but the file is silently skipped. This should be the default behavior when the --allow-missing-files flag is set. However, even without the flag, currently the processing just emits an error message, but continues. This is especially annoying when running in the grid and, thus, getting the error messages written into log files instead of to console.

The problem is that the continue: https://gitlab.idiap.ch/bob/bob.bio.base/blob/master/bob/bio/base/tools/extractor.py#L129 is not what we want. I think, when the processing fails and None is returned, we should raise an exception rather than skipping.