Suggestion: Syntax errors for malformed `extern` statements could be clearer

[Wilfred]

This is a very minor bit of polish, but I bumped into it. Given the following syntactically invalid code:

extern crte hyper;

fn main() {
    println!("hello world")
}

rustc produces the error:

$ rustc foo.rs
foo.rs:1:8: 1:12 error: expected `{` or `fn`, found `crte`
foo.rs:1 extern crte hyper;
                ^~~~

Ideally, the syntax error message would suggest crate in this instance too.

Whilst there are other tokens that can occur after extern (such as strings), I think crate is sufficiently common to warrant an explicit mention.