Request: Clarify error message when NAs present in intervalaverage args

[lpiep]

NAs in interval_vars leads to this error: columns corresponding to interval_vars cannot be missing in x (or y)

What would you like to see happen?

This should be clarified to say NAs are not allowed in these columns. It is otherwise confusingly similar to the error where the specified interval_vars columns just don't exist in the input.