bad type signature (with splat) could have a more clear error message

[adienes]

in my opinion this would be better as a syntax error than an internal error

julia> bar(t::Tuple{X...}) where X = t
ERROR: MethodError: no method matching iterate(::TypeVar)
The function `iterate` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  iterate(::LazyString)
   @ Base strings/lazy.jl:94
  iterate(::LazyString, ::Integer)
   @ Base strings/lazy.jl:95
  iterate(::CompositeException, Any...)
   @ Base task.jl:55