Show clearer error for .docx mode

[dstillman]

https://forums.zotero.org/discussion/82193/zotero-experienced-an-error-updating-your-document

[adomasven]

@dstillman please check the copy at https://github.com/zotero/zotero-connectors/commit/8342bb52d349f9e00f0d557c0ee90b533f4e5aec