melt error message for bad measure.vars could be more informative/specific

[tdhock]

Right now we have the following error message, which is ok:

> melt(data.table(x1=1,x2=2,y1=3,y2=4),measure.vars=c("x1","x2","z1"))
Error in melt.data.table(data.table(x1 = 1, x2 = 2, y1 = 3, y2 = 4), measure.vars = c("x1",  : 
  One or more values in 'measure.vars' is invalid.

it would be even better if we could tell the user specifically which entries are invalid, and how to fix the issue, for example "One or more values in 'measure.vars' is invalid; please fix by removing [z1]"