[References] Editing a Zotero reference without 'firstName' and 'lastName' for 'creators' makes NeuroCurator crashes

[pafonta]

Steps:

1. Click on a reference using 'name' instead of 'firstName' and 'lastName' for 'creators'.
2. Click on 'Cancel' if the 'This paper is not already in the curator database' dialog appears.
3. Click on 'Zotero' in the menu bar.
4. Click on its item 'Edit reference'.
5. NeuroCurator crashes with the following stack trace.

The only reference causing this issue is the one with the DOI 10.1113/JP274049, as of 20.08.18.

This issue is related to the discovery made in BlueBrain/nat#17.

Stack trace:

Traceback (most recent call last):
  File "/Users/fonta/repos/neurocurator/neurocurator/zotero_widget.py", line 148, in edit_reference
    dialog.load_reference_data(reference["data"])
  File "/Users/fonta/repos/neurocurator/neurocurator/zotero_edition.py", line 84, in load_reference_data
    widget.load_creators(ref_data[self.CREATORS_FIELD])
  File "/Users/fonta/repos/neurocurator/neurocurator/zotero_edition.py", line 220, in load_creators
    cell_value = creator[self.HEADERS[j]]
KeyError: 'firstName'