Generate clear error when a nameless contract argument is used

[sisuresh]

What

Resolve #1198

What the error looks like now -

error: argument not supported
 --> src/lib.rs:9:18
  |
9 |     pub fn empty(_: u64) {}
  |                  ^

error: argument not supported
 --> src/lib.rs:9:28
  |
9 |     pub fn empty(dog: u64, _: u64) {}
  |                            ^