common: Log warning for invalid JSON Schema in which "properties" values aren't JSON Schema

[jpmckinney]

Fixes recent Sentry errors in cove-ocds and kingfisher-process.

Occurs because an MTender extension does:

    "ValueDifference": {
      "title": "",
      "description": "",
      "type": "object",
      "properties": {
        "value": "",
        "unit": {
          "$ref": "#/definitions/Unit"
        }
      },
      "extension": "eAuction"
    },

value can't be a string. It must be a JSON Schema.

Problem is in https://raw.githubusercontent.com/eOCDS-Extensions/eOCDS-eAuction/master/release-schema.json

cc @duncandewhurst