UI messages for JSON Schema validator==additionalProperties needed

First used in Open-Telecoms-Data/open-fibre-data-standard#274

Every type of error has a header and a description, so we need those written for this error.

Needs to be added to:

• https://github.com/Open-Telecoms-Data/cove-ofds/blob/live/cove_ofds/jsonschema_validation_errors.py
• https://github.com/Open-Telecoms-Data/cove-ofds/blob/live/cove_ofds/templates/cove_ofds/jsonschema_validation_panel.html

[duncandewhurst]

Added to the CoVE OFDS jsonschema error messages spreadsheet.