warpSet: check grouping suggestion

[SamGG]

I think grouping could be checked at the beginning of the function. This would avoid getting error after some computation started.

Lines 657-659
if(!is.null(grouping)){
if(!grouping %in% names(pData(x)))
stop("'", grouping, "' is not a phenoData variable.")
could be placed around line 593.

Best.