Nonsensical macro_rules error message

Consider the following (malformed) macro definition:

``` rust
#![feature(macro_rules)]

macro_rules! foo( (i: ident) => (
        i[0] = 7;
    );
)

fn main() {
    let mut i = vec!(0u, 1, 2, 3);
    foo!();
    println!("{}", i);
}
```

Compiling this fails with:

```
<anon>:1:1: 1:1 error: unexpected end of macro invocation
<anon>:1 #![feature(macro_rules)]
         ^
```

This isn't helpful.
