Make argument type error message consistent for variadics

Make argument type error message consistent for variadics

If an argument error refers to a variadic argument, we normally
do not print the name of the variadic (as it is not referring to
an individual argument, but to the collection of all of them).
However, this was not the case for the userland argument type
error message, which did it's own formatting.

Closes GH-6101.