[Rd] 'unique' error message is printed despite silent=TRUE (PR#13547)

[macrakis at alum.mit.edu]

In 2.8.0/Windows Vista:

When 'unique' gives a type error message, it prints out even if errors
are being caught:

> try(unique(quote(hello)),silent=TRUE)
hello

This comes from the .Internal unique routine:

> try(.Internal(unique(quote(hello),NULL,NULL)),silent=TRUE)
hello

I guess it is using the internal equivalent of print rather than the
internal equivalent of stop.

           -s