Make non-unique keys message more informative

[robne1982]

If I run

rCompare(ddf, ddf2, keys = c("a", "b", "c", "d","e"))

but there are duplicates in c("a", "b", "c", "d","e") I get an error message like:

Running rCompare...
Error in matchSingleIndex(df_a, df_b, "dataCompareR_merged_indices", indices) : 
  The indices are not unique in the submitted dataframes. Please resubmit with unique indices.

I expect my indices are unique, so now I have to write some code to figure out why they're not!

I'm guessing the code knows more about what element is not unique, so it'd be nice if it told me!