unresolvable prev/next meta should lead to warning not crash

[amir-zeldes]

Ingesting a document with prev/next urn that does not actually exist in another document leads to a successful ingest, but the html vis for the document is not accessible. Better behavior would be:

• Ignore prev/next (behave like the document just doesn't have that field
• Output a warning on the cli during ingest

[lgessler]

relevant trace:

  File "/usr/local/lib/python3.6/dist-packages/django/core/handlers/base.py", line 124, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/var/www/cts/coptic/coptic/views.py", line 47, in text_view
    visualization = text_object.html_visualizations.get(visualization_format__slug=format)
  File "/usr/local/lib/python3.6/dist-packages/django/db/models/manager.py", line 82, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/usr/local/lib/python3.6/dist-packages/django/db/models/query.py", line 399, in get
    self.model._meta.object_name
texts.models.HtmlVisualization.DoesNotExist: HtmlVisualization matching query does not exist.

[lgessler]

Looks like the logic for this is already in place: https://github.com/CopticScriptorium/cts/blob/master/coptic/coptic/views.py#L57L68

This makes sense since it wasn't the fact that the value of next wasn't a valid document_cts_urn which was causing the exception I pasted above. There must be some subtle interaction between having a bad next and that stack trace above. (They seem connected, since fixing the value of next made the issue go away.)