Check NAs when m is constant and throw meaningfull error message

[psolymos]

Cases are dropped due to NAs will lead to mismatches in used/avail and ID vectors because objective function can't make a nice rectangular data of the available points.

Catch this case and throw a meaningful message (i.e. metching is OK before cases are dropped, but not after): "Mismatched IDs due to missingness: check NAs in data"