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.