extract_text: Add basic exception handling
extract_text: Add basic exception handling

This should handle the case that the `Document.data` of a given row is
null, as well as any errors that may arise from within the call to
`bytes_to_text`.

Currently, Rows where an error occurs aren't filtered out of the
results dataframe, though that could be added.

opensyllabus/osp-pipeline,opensyllabus/osp-pipeline