Confusing error messages with ||-closures vs binary "||"

```
fn foo(f: fn()) { f() }
fn main() {
    foo || {}; 
}
```

produces:

```
shit.rs:4:4: 4:13 error: binary operation || cannot be applied to type `extern fn(fn())
```

It'd be nice if it checked if the thing on the left had type "fn(fn \* -> *) -> *", and said "Did you mean `do foo || ... ?`"
