message: [Azure Search] Add indexer warnings to latest preview API spec
diff: "errorMessage": {
  "type": "string",
  "readOnly": true,
  - "description": "The message describing the error that occurred while attempting to index the item."
  + "description": "The message describing the error that occurred while processing the item."
  }
  },
  "description": "Represents an item- or document-level indexing error."
  },
  + "ItemWarning": {
  + "properties": {
  + "key": {
  + "type": "string",
  + "readOnly": true,
  + "description": "The key of the item which generated a warning."
  + },
  + "message": {
  + "type": "string",
  + "readOnly": true,
  + "description": "The message describing the warning that occurred while processing the item."
  + }
  + },
  + "description": "Represents an item-level warning."
  + },
  "IndexerExecutionResult": {
  "properties": {
  "status": {
  },
  "description": "The item-level indexing errors."
  },
  + "warnings": {
  + "type": "array",
  + "readOnly": true,
  + "items": {
  + "$ref": "#/definitions/ItemWarning"
  + },
  + "description": "The item-level indexing warnings."
  + },
  "itemsProcessed": {
  "x-ms-client-name": "ItemCount",
  "type": "integer",
  