Search: when one line fails to index, the whole page is skipped

Example, here a line has no baseline and no mask:

Failed to index part 2284 on project 4 because: 'NoneType' object is not iterable
Traceback (most recent call last):
  File "/home/robin/Projects/escriptorium/app/apps/core/management/commands/index.py", line 117, in index_project
    total_inserted += self.ingest_document_part(
  File "/home/robin/Projects/escriptorium/app/apps/core/management/commands/index.py", line 179, in ingest_document_part
    "bounding_box": [ceil(value * factor) for value, factor in zip(line.get_box(), scale_factors)],
TypeError: 'NoneType' object is not iterable

Such lines shouldn't exist but we should try/except on the line level anyway to avoid skipping the whole page for unforeseen errors and invalid data.