Make argument type error message consistent for variadics

[nikic]

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.

[nikic]

@kocsismate This is the intended behavior, right?

[kocsismate]

@nikic TBH I like that variadic parameter names are displayed, just if all the passed variadic arguments would have the same name :) But the option you've just implemented makes sense for sure, consistency seems more important.