Fix span on unknown macro variable error

[emberian]

In #6686 I added an error, but the span is incorrect. Ideally it would cover the use of the unknown variable and not the m in macro_rules!.

erickt | maybe the caller of lookup_cur_matched has the span you are looking for?
erickt | if so, you could pass it in and use that for your error message

[graydon]

Reproduced 2013-07-25. Relative of #7970 perhaps? Nominating for production ready.

For ease of future reproduction, here's the test. Note where the failure is pointing to, relative to where the actual error is (the use of $nonexistent):

macro_rules! e( //~ ERROR unknown macro variable `nonexistent`
    ($inp:ident) => (
        $nonexistent
    );
)

fn main() {
    e!(foo);
}

[catamorphism]

Accepted for production-ready

[pnkfelix]

P-low, not 1.0.

[steveklabnik]

This seems to have been fixed:

hello.rs:3:21: 3:33 error: unknown macro variable `nonexistent`
hello.rs:3                     $nonexistent
                               ^~~~~~~~~~~~