Fix error message

Number of groups was sometimes incorrect.

src/extras.jl

@@ -268,9 +268,8 @@ function cut(x::AbstractArray, ngroups::Integer;
     breaks = quantile(xnm, (1:ngroups-1)/ngroups)
     breaks = [min_x; breaks; max_x]
     if !allowempty && !allunique(@view breaks[1:end-1])
-        n = length(unique(breaks)) - 1
-        throw(ArgumentError("cannot compute $ngroups quantiles: `quantile` " *
-                            "returned only $n group(s) due to duplicated values in `x`. " *
+        throw(ArgumentError("cannot compute $ngroups quantiles due to " *
+                            "too many duplicated values in `x`. " *
                             "Pass `allowempty=true` to allow empty quantiles or " *
                             "choose a lower value for `ngroups`."))
     end