Improve error message when shard key not present in all documents

The checkShardingIndex command returns the following error message if any of the documents in the target collection do not contain the shard key fields:

"code": 96,

"codeName": "OperationFailed",

"errmsg": "found missing value in key { : null, : null }"

This error message should be optimized for user-education and next steps. For example it should probably say "Documents missing shard key fields - ensure all documents are updated to include all fields" or similar.