[data grid] Refactor rowCount error to a warning

[michelengelen]

Is there a deeper reason for this error message or just to warn the developer @michelengelen?
We don't use server side fetching, but use filtering outside of the Datagrid. So, there's a subset of the total rows supplied to the grid in some occasions. We use rowCount to display the total number of rows in the footer.

Now we get an annoying error message. We know it doesn't have any "effect", but we need it for display reasons anyways. Unless there's an actual error, maybe warning would be more appropriate (and only in dev), otherwise, it's unnecessary code that gets bundled into production it seems?

This is a valid request IMO ... changing this to a warning also does not hurt as there isn't an actual error to be expected here.

Originally posted by @lauri865 in #12448 (comment)

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